@Elle19 - If you are going to require a guest to sign a rental contract, you need to present that before the guest makes their reservation. You must also declare that the guest must sign one in your listing A host has a right to cancel without penalty if they are not allowed to review the terms of agreement before booking. https://www.airbnb.ca/help/article/465/can-hosts-ask-guests-to-sign-a-contract
This pretty much means you have to turn off Instant book or list the terms in your house rules. I've seen both options with guests requiring a separate and unique contract. Please note, Airbnb will not enforce your contract nor endorse it.
The easiest thing to do is have the guest sign it when they arrive. You could also invest in Docu-sign and send it electronically for signature after the booking is confirmed.