Follow the link in your confirmation email and sign in with the same email address.
Your confirmation email includes a link to your booking page, which shows your dates, guests, pitch, extras, price breakdown and any balance left to pay.
You'll need to be signed in with the email address you booked with. Signing in just means entering a six-digit code we email you; there's no password. If you booked without an account, see I booked without signing in. How do I claim my booking?.
Email support@pillowcase.io with your campsite name or booking reference and we'll pick it up from there.