Stickers modify cards; they are found on a set of sticker sheets and persist on the cards as they move between public zones. Stickers do not need to be attached directly to cards; they can be attached to card sleeves or associated in any way that makes the function of the sticker clear.
Stickers were introduced with Unfinity and are now an unironic part of tournaments. While stickers are placed on cards, protective coverings around the cards are fine. And, as discussed below, they don’t technically need to be attached at all. They can be placed on the card like a counter
Sticker sheets are identified in deck registration using a combination of the three name stickers on the sheet. Players present their registered sheets of stickers alongside their deck during the pregame procedure and, if necessary, determine the three sheets to be used for that game at that time.
The three names on a sticker sheet is sufficient to uniquely identify it for the purpose of registering your stickers. While you still register stickers on your deck list, it is not a supplementary deck like is used for attractions. You do not have access to your full set of stickers, during gameplay, but rather a random selection of three sticker sheets determined during pregame procedures.
If a player has not registered a set of sticker sheets but is instructed to put a sticker onto a permanent they own due to an effect they have gained control of, they may visit https://magic.wizards.com/en/unresources to generate three sticker sheets for use in that game only. If they do not choose to do so, the part of the ability instructing them to put a sticker onto a permanent is ignored.
It is possible, during the course of play to be told to place a sticker on a card when you did not prepare for stickers. In that case the game creates three random sticker sheets for you, in the same way the game creates dungeons if you somehow have to venture into a dungeon. The website listed above will generate a list of 3 random sticker cards from the list of real sticker sheets. You may then use dice or counters of pieces of paper to represent the stickers you use. If you don’t want to mess with stickers, you can just ignore the instruction.
Official sticker sheets are not required, provided that the player makes it clear what sheets they are using, which stickers are on those sheets, and represents the stickers in-game with a method that is clear to both players. Only the official 48 sticker sheet combinations may be used; players cannot mix and match to generate their own sheets.
fWhile real stickers are preferred, the game doesn’t actually require you to physically have the physical sticker sheet. You can photo copy them and put them in sleeves for random selection. The important part here is tracking which stickers have/have not been used when using sticker proxies like dice or paper.
All stickers are returned to their respective sheets between games.