Perks can only be used in certain Rooms. If available, the game will automatically offer you a selection of suggested Perks at the start of a run. Just pick any or all that you wish to use to improve your progress!
If you already own a specific Perk item, you can activate it during your run. If you don't own a Perk, you can always pay Tokens to activate it instead.
If you already own a specific Perk item, you can activate it during your run. If you don't own a Perk, you can always pay Tokens to activate it instead.