1. DECK CONSTRUCTION.
1.1 Deck Size. An Infinite Magic deck should have between 80 - 150 cards, though this is a recommendation not a rule. I've found that if you build your deck with under 80 cards, each game plays out similarly with everyone hoping to get a specific card or combo. If you try building your Infinite Magic deck with over 150 cards - it just gets unwieldy (particularly if you've double sleeved your cards). There's no prohibition against smaller or larger decks, but once you play a few times, you'll understand what I mean.
1.2 Deck Contents. The main thing to remember is that you don't have to include land in your Infinite Magic deck. You can if you'd like. I've got a Maze of Ith in one of my Infinite Magic Decks, and I'm sure the creative among you will think up all kinds of broken uses for lands in a game where you have infinite mana. That said, most beginning Infinite Magic decks will probably be 75% / 25% creatures and spells, respectively. But what you end up brewing is really up to you and your playgroup.
1.2.1 Suggestions.
1.2.1.1 I recommend that you don't include tutors. They slow the game down. And if you've done a good job building your Infinite Magic deck, most of the cards will be bombs already.
1.2.1.2 Balance is important. Don't put in a Savannah Lions and an Avacyn, Angel of Hope. The person drawing the Savannah Lions will not be having a good time. But do feel free to put in an Avacyn, Angel of Hope - balanced with a Pathrazer of Ulamog. (After all, you have infinite mana, so Annihilator 3 won't wipe out someone's ability to play the game.)
1.2.1.3 Know your Audience. As a casual format, Infinite Magic is designed to be played in groups and with strangers. That said, if you've built a "flickering planeswalkers" Infinite Magic deck, maybe don't subject new Magic: The Gathering players to quite that level of complexity. M'kay?
1.3 There are no banned cards - other than the caution that 'X' spells can break your game. (The 'power nine' [with the exception of Time Walk and Time Twister] are spectacularly useless when you have infinite mana - and there are plenty of 4 - 7 CMC cards that duplicate what the blue power nine do.)
1.4 'X' spells. In general you shouldn't include them. Since you have infinite mana anyone holding an 'X' spell (or creature with firebreathing, etc...) can one-shot another player. If you include any 'X' spells, make sure you make that very clear to your playgroup. (For example, I have an Infinite Magic 'dragons' deck, and I include Bladewing the Risen since beating your opponents down with a legendary zombie dragon is just a cool way to win. But I tell everyone who plays with me that Bladewing is in the deck before we start playing.)
1.5 Deck Building Strategies. The sky's the limit here. But you should carefully consider each card you add to your Infinite Magic deck. Experienced Magic: The Gathering players will understand what I mean. For example, I'm a huge Thraximundar fan, but I've removed him from my Infinite Magic decks because he can easily lock another player out of the game if played on turn one and no one has removal in hand. Want to make a planeswalker Infinite Magic deck? Have at it, but maybe consider _not_ adding Oko, Thief of Crowns?
2. RULE ZERO CONVERSATION.
2.1 Much like a game of Commander, you should explain to your playgroup what kind of game they can expect to have if they sit down to a game of Infinite Magic with you. For example - perhaps you should tell your mates that your Infinite Magic deck is 90 counterspells and 20 creatures. (And if you actually build that deck, you are a terrible person and you should feel bad.)
3. OPENING AND MAXIMUM HAND SIZE.
3.1 Shuffle your Infinite Magic deck and deal each player 3 cards.
3.2 Mulligans. As a casual 'for fun' format, I'm all for allowing multiple mulligans. I recommend that each player can take 1 mulligan, and anyone who didn't build the Infinite Magic deck you're playing from can take 2 mulligans - each with no decrease in opening hand size. But, you can 'house rule' this if your playgroup is OK with it.
3.3 Normally, in a multiplayer game, each player would draw on their first turn. This is not the case in an Infinite Magic game. In an Infinte Magic game, each player skips their first draw.
3.4 Maximum hand size in an Infinite Magic game is 6. You can 'house rule' this if you'd like, but I've found that 6 is more than enough to i) give players a decent amount of choice and card variation; and ii) prevent indecision paralyzation.
4. RULES OF PLAY.
4.1 Each player starts with 20 life. You can 'house rule' this if you'd like, but I've found that 20 is appropriate for the Infinite Magic format.
4.2 Choose someone to go first, and play proceeds clockwise. In our playgroup the first person outed in the previous game gets to go first. But feel free to roll dice, perform ritual sacrifice, or whatever else everyone agrees on. Just remember rule 3.3, which is that no one draws in the first round.
4.3 Cards that refer to 'your' graveyard or library refer to the communal library and communal graveyard respectively. For example, if someone tucks a card "3rd from the top of your library", that means the card is tucked 3rd from the top of the communal, Infinite Magic deck. Similarly, if a card says "return target creature from your graveyard to your hand", you would return target creature from the communal graveyard to your hand.
4.4 You have infinite mana. As such there is no need to tap lands (or even include them in your Infinite Magic decks.) You may generate in infinite amount of colored (any color or combination thereof) or colorless mana at any time you are normally be permitted to do so under the rules of Magic: The Gathering.
4.5 You may only play one spell from your hand each turn. Note that playing spells from a 'cascade' effect or that have been exiled with a card like Light Up the Stage do NOT count as playing a spell from your hand. So be careful when adding these types of cards to your decks. Note also that if you have a hand full of instant spells, you can play one EVERY turn if you'd like.
5. NUMBER OF PLAYERS.
5.1 Infinite Magic is designed to be a multiplayer format. I recommend 3 - 5 players. Infinite Magic can be played 1 v 1, but that eliminates the potential for political maneuvering. And if you have 6 or more players, it can get unwieldy. (And if you have 6 players, just split into two pods of 3 - or hell, try Two Headed Giant Infinite Magic.)
6. EDGE CASES AND ONE-OFFs.
6.1 In most respects playing a game of Infinite Magic is just like playing a faster paced game of multiplayer Magic: The Gathering. But there will be edge cases and one-offs. The golden rule here is that this is a format for fun, not pub-stomping. Use your reasonable discretion and do what seems like the most fun for the whole table.
6.2 And finally, look to the general rules of Magic: The Gathering if you have questions about timing or anything else not covered here.
