If, after reading this, you're still nervous, and attending a game is the most important thing to you, go with a tour. Your hotel definitely has one. Or you could hook up with Sergio, a wonderful guide. I didn't use him myself, and I've never met him; we've only traded email. But he was helpful even though I wasn't a customer, and demonstrated a genuine affection for the game. I got the clear sense that with him, I wouldn't be getting an overly packaged experience. And his rates were lower than the hotel's.
But I hate tours, and wanted to do this not surrounded by a group of other terrified tourists. So what follows is some approximation to step-by-step instructions on how you can do this for yourself.
Two more things to note.
Maracanã isn't the only stadium in Rio. Engenhão is another major (new) stadium with a full slate of games. But it's a bit farther from the subway, and I wasn't as sure about its neighborhood. Nevertheless, I imagine it makes for an equally exciting venue. (And you can get bragging points: “Oh, these days everyone goes to Maracanã, but I...”. You could make it out to be the K2 of Rio stadiums at your pub back home.)
More importantly, I attended two mid-season club games with only one team from Rio. So everything was easy. None of this applies for games whose results matter more, when a famous rivalry (such as Fla-Flu) is involved, when the national team plays, etc.
Okay, let's begin.
You can find a schedule to games on the CBF site. As of this writing, click on “Série A” at the top, and try the links on the left. “Escalas” should give you a schedule, but it may only show the current week. This link gave me a full season schedule, but sometimes the Web page just produces an error. This kind of difficulty may be good preparation. [Note: Sergio's site, linked above, usually contains the game schedule.]
Warning: I made various game plans based on this schedule. I sent email to two Brazilian friends to confirm I'd read everything right, and I had. A week before the games I checked the schedule again, and every game had changed in some way (time, date, ...). So give yourself a little flexibility, and check again closer to the date.
The stadium's name is not Maracaña—it's not in Spanish. It's pronounced “mah-RAH-ka-na”. You may hear the metro announcer pronounce it as “mah-RAH-ka-nu”.
Learn a few key words: today (hoje: ho-ZHAY), tomorrow (amanhã: ah-ma-NYA), yellow (amarela: ama-RAY-la), green (verde: vehr-DJEH), white (branco: br-AHN-cu). Practice the pronounciation a bit: though the written words are very similar to Spanish, they aren't spoken quite the same way. If you're taking my suggestion on tickets, and all this language stuff terrifies you, you could write a note containing the date, names of teams, “arquibancada verde/amarela”, and a number (of tickets), and slide it in the ticket window. This has the advantage that you will almost certainly have no problem at all, and the disadvantage that you will have failed as a traveler. [If you are repelled nevertheless, it may be because you're trying to buy tickets for a future game—even the next day's—and they aren't on sale on the current day.]
Research the team colors. The Wikipedia pages for all the teams I saw gave their home- and away-colors. You would do well to avoid wearing any team colors at all. (Admittedly, the second time I accidentally wore a shirt in partial team colors—one of Fluminense's tricolor—and nobody seemed to notice or care. Still, standard precautions apply.) And from reading those pages, you may also learn a chant or two.
You can usually buy your ticket the evening of the game—various sources recommend getting there two hours ahead. I found it easier to go earlier in the day, to keep my afternoons flexible. The ticket office seems to be open at reasonable hours. (You can also buy the tickets directly from the team's box offices—I located the one for Flamengo on Rua Raul Machado, two blocks west of the Lagoa Rodrigo de Freitas (that's right, it's not in the Flamengo section of town)—but going to buy the ticket is a good dress-rehearsal for getting to the match.)
To get your ticket, take the metro to the Maracanã station. You can't miss the stadium; you really, absolutely cannot. (The only way you could miss it is if visibility were under a hundred yards—if it is, what are you doing at a football game?) When you emerge from the station, take the (can't miss) ramp to the left. This will put you at one of the stadium's main gates. Facing the gate, proceed left along the stadium wall. You'll soon get to the ticket window. There seems to always be a handful of people milling about (including touts, who left well enough alone when I spoke in Portuguese but hassled people who spoke English). Also look for the word Arquibancada above the little teller windows. This is the word for the upper floor of the stadium, and where you want to sit.
Buy your tickets. I asked for Arquibancada Verde, the green stand; one time I was given a ticket for it, another time for the amarela (yellow) section. They're essentially indistinguishable. Those tickets were BRL 30. For more, you can sit in the white or blue sections, where you sit side-on to the field. On the back of your ticket you'll see all the relevant details (stand, date, time, teams) printed, so make sure these are what you wanted.
In terms of time, using the metro, it never took me longer than about an hour to get to or from the stadium, and that's from Cantagalo, currently the end-station of the other line (in western Copacabana). I expect 1h15m is a very safe estimate. I once got there in just about 45 minutes.
There is no real in-stadium pre-game tradition, nor the equivalent of batting-practice. So there's not much to do if you get there early, other than revel in the fact that you're there. Which is something in itself, so you might as well. Be aware that the locals seem to mill about outside the stadium until just before the game, so your “empty” section may end up packed. It may even be that you ended up in the heart of a team's cheering section, with drums behind you and flares going off overhead. This is not a hypothetical.
Why do people mill about outside? In part to meet their friends, etc., but I think mainly because you can't buy alcohol inside the stadium. The only beer is alcohol-free (if you order a beer and are told something that sounds like a disclaimer, that's what they're telling you). So you'll see lots of vendors selling alcohol outside, including on the ramp between the metro and the stadium. Get your fill if you must while you can. I chose to not dull my senses. (And given the dismal quality of Brazilian beer, the non-alcoholic stuff in the stadium was no great loss.)
You may have noticed that your ticket has some slightly bewildering code indicating your actual seat. For the games I went to, they weren't bothering with assigned seating. It meant I was free to roam around the stadium, and indeed I periodically moved between stands to get different views of the stadium, the game, and the cheering sections. If they are checking seating assignments, then I expect you will simply be pointed in the right direction.
You will be searched before you go in—a quick and friendly pat-down. (As you walk up the ramp in the stadium, you'll see guys lifting their shirts. They're showing the police their belts, though you may think this is just beach machismo gone awry.) I believe I saw backpacks get in; they didn't blink at (and certainly didn't inspect) my umbrella. I expect a bottle of water is also fine (they aren't obsessed with stadium concessions like they are in the US), but do leave the heavy artillery at home.
You won't actually find much at the stadium concessions. There's no real food to speak of. I saw some vendors selling what looked like boxed, pre-made hot-dogs; there were various kinds of snacks; and that's about it. On the other hand, it's not very expensive. (Beer was BRL 4, chips and such about BRL 3, a little bag of nuts is BRL 1. So you don't need to carry much cash.) One warning: the nuts (amendoim) contain monosodium glutamate (MSG).
One other tip. If you don't already have one, buy your return metro ticket before the game. (When people get off the metro they're all focused on finding friends, etc., so there are no queues at all to buy tickets.) This will save you a lot of waiting later.
My first game began at 8:30, so I got back to Cantagalo sometime around 11:30pm. It would be false to say that walking the 5-6 blocks back to my hotel (a block off the beach) felt like “the safest thing in the world”, but it did feel very safe. Even the streets immediately around the station, which are a bit dark, are peopled. They're all working-class folk, many of them enjoying what is presumably a post-work drink at the little local bars and snack counters, away from the tourist places. But because they're locals, not tourists, they do know their colors, and may be a bit tired or tipsy. So this is one place where wearing team colors may just cause a bit of trouble.
If you want a team jersey, the official ones cost a pretty penny. You can find cheap ones, but these aren't quality prints, and are presumably not legal. Anyway, if that's what rocks your boat, you can even find them outside the stadium. I approached a vendor and was quoted BRL 30. I laughed, and he immediately dropped it to BRL 20. I tried to talk him into BRL 16 and he wouldn't take it. Okay, that gave me a lower-bound.
Later, I walked around in the street-market in Copacabana and asked some of the guys selling Brazil team shirts for local team shirts. I noticed that asking them somewhat loudly made them immediately say no (so these are illegal!). But if you linger at a store for a few moments a store-keeper will eventually approach you; mention the team you want to him in a low and conspiratorial voice. He'll take you to the back of the store-tent and pull the jersey out of a big, black trash bag full of illicit team jerseys. He too will start with BRL 30-32, so just say “I can get this at the stadium itself for ...!” He'll fold right away. And you're probably still paying way too much for a cheap rip-off. I did.