Put it in the important notes of your booking emails.
Pillowcase doesn't enforce cancellation policies automatically; cancellations and refunds are always in your hands, so your policy is whatever you tell guests it is. The important notes field in your booking emails (Settings, Emails) is the place to state it, so every guest has it in writing from the moment they book.
It's worth putting the same policy on your own website too.
Email support@pillowcase.io with your campsite name or booking reference and we'll pick it up from there.