The "Instant" card type and the "Flash" keyword are removed. If a card has the "Flash" supertype, they can be played at any time you have priority. (Like a card with the flash keyword).
Like official formats, players roll dice (or otherwise use chance) to decide who goes first. If the player who wins the die roll draws their hand before choosing whether to go first or second, they are forced to choose going first.
Whichever player takes the first turn of the game may mulligan one time 'for free', without going down to 6 cards in hand. You must still mulligan to 6 cards or fewer to scry 1 after mulligans.
Whichever player takes the second turn of the game begins the game with one colorless mana in their mana pool. It doesn't empty from their mana pool as steps and phases end. (This mana is normally represented by a token.)
The 'legend rule' present in official formats is removed. Players can control multiple Legendary permanents with the same name, without being forced to sacrifice all but one. HOWEVER, players may only include one copy of any given legendary card in their deck. (Players can still include multiple different legendary cards in their deck. Players cannot have a copy of a given legendary card in both their library and their sideboard.)
The last step of a library being searched is for it to be shuffled by its owner. 'Shuffle your library' is left off of cards that search your library in TumbledMTG, and it's simply assumed it must always be done by the library's owner.
If a nonsorcery (ie, creature, enchantment or artifact) spell is copied, the copy will behave much like you would expect a normal spell of that kind to behave. When it resolves it will be put onto the battlefield as a token permanent.
If an ability can only be activated at 'nonflash speed', that means it can only be activated any time you could cast a nonflash spell.