For many playtest cards, you’ll need to make a generous assumption that basic game rules would be updated to allow them to work. The Mystery Booster 2 Playtest Card Notes section (reproduced here in individual Gatherer rulings) provides guidance for fitting these cards into the existing rules structure.

After a creature leaves the battlefield, it does briefly enter whatever zone it was put into before it ceases to exist. For example, a creature that’s put into a graveyard and then ceases to exist still triggers “Whenever a creature dies” abilities.

Playtest cards use a modified version of game symbols, such as and . These modified symbols should be treated as the standard symbols during play.

Playtest cards aren’t legal for play in any tournament format other than Mystery Booster Limited formats. On the other hand, we expect they will spice up a wide variety of non-tournament games (as long as everyone’s on the same page about using them!).

Tokens aren’t cards.

Creatures that cease to exist aren’t put into exile. They just stop existing. Put them somewhere separate so it’s clear they no longer exist. (I can’t help you with that existential conundrum—I’m just a humble rules person.)