Once you choose a guest name or register (just a name and password, no email), the game immediately throws you into a room. It can be very disorienting, since even if you click on the "help" button; it doesn't fully explain all the controls, strategies, and secrets of the game. There are over sixty maps, and on each map there is at least one hole and at least one piece of cheese. Your goal normally is to get cheese and get to a hole, and you do this with the [WASD] or [arrow] keys to move and jump.
One or two mice every round are chosen to be shaman. The shaman can summon objects near himself and use the "spirit" command to make mice jump farther. He does this by clicking on an object in the lower right, then summoning it within his range. But he must also consider whether he wants to rotate the object (mouse wheel, [Z] and [X], or [ctrl] and [shift]), whether he should anchor it ([C] is a free anchor to another object, [V] is a fixed anchor to another object, [B] is a fixed anchor to nothing, and [N] is a rotating anchor) and whether it should be invisible ([spacebar]). Then there's the sheer variety of things you can place on levels, from trampolines to cannonballs to anvils.
As even the instructions show, being a shaman is complicated, and your first try as shaman will probably consist of the other mice screaming at you via the chatbox at the bottom of the screen UR DOIN IT RONG, only probably less polite. So you'll probably want to change room to a room with no one in it and practice your skills. Type /room and then a number or word to change to that room, and if no one is in that room, you'll be the shaman every time.
Analysis: Transformice seems to be a game that is well aware of the various motivations for people to play. Some multiplayer games get utterly broken by trolling and vicious players because the gamemakers obviously didn't anticipate that playstyle. Transformice not only anticipates it, some levels seem to encourage it: in the quickly developing lingo of the game, duelling maps are where two shamans have two color coded holes, one for each shaman. Generally the first thing that happens on these maps is the shamans try to destroy each other or block each others holes. In fact, the game seems to take a lot of gamebreakers for granted. For example, some of the maps seem to be expecting you as the player to try for clipping errors.
On most of the levels, the best result for you, personally, is to be cooperative, and in many levels trying to sabotage others frequently results in your own death. While messing around occasionally is usually taken in good humor, true trolling as a shaman is very badly received in most rooms. You may be banned by the other players (/ban playername), other shamans may single you out for killing when the turn switches, or you may simply find that your audience simply scampers off to other rooms.
For me, I had the most fun playing when someone suggested in group chat going to a room composed just of people who want to earn cheese. A bunch of us did, and for a good hour we racked up cheese very quickly because we were all cooperating. Here's the funny thing, and it will help you understand why a game of adorable cartoon mice is rating-r: I can't tell you the name of the room we used because Jay would wash my mouth out with soap. Swearing and vulgarity are near constant in the chat boxes of Transformice. So are memes: when some epic fail necessitates becoming an hero, the most frequent shout is "SPARTAAAA!" (If you understood all the references in that sentence, you'll fit right in.) If someone is being particularly offensive (slurs are a deal breaker for me), you can click on their name and ignore them.
Never forget about the option to change rooms. Different rooms have different climates, which develop as people choose to stay or leave. In the total opposite of the room where I had the most fun, sometimes a room will develop in which every single person is using their term as shaman to try to kill everyone else, and every single person is having a lot of fun doing this. Ask yourself what kind of play style you like, and then try different rooms until you find one with people who like it too.
Take advantage of the ability to create rooms! (Be aware that no one owns a room, and anyone can enter.) Try out /room jig to hang out with out JayIsGamers. How about /room jignoobs for a more forgiving place where you're all learning, /room jigevil for a place where everyone tries to kill everyone, and /room jignice for people who want to be sweet and helpful? Do you have more ideas for themes for rooms? Suggest rooms and tell us your Transformice nickname in the comments! Really great Flash multiplayer games are not easy to come by, and the addictive and chaotic gameplay of Transformice ranks with the best. Plus, cheese. Oh yes.
Thanks to SirNiko, B, Teasel, and Pil for sending this one in!