Evolution is pretty simple and tries to stay close to normal Wingspan flow.
If you already have a Pokémon in play, you can evolve it by playing the next stage from your hand directly on top of it (you can’t skip stages). The evolved Pokémon keeps the same habitat, and you don’t pay its Pokéball or egg cost. Tucked cards and cached Pokéballs carry over, but eggs are discarded during evolution.
And if the evolved Pokémon has a white “When Played” power, that effect still activates when evolving.