Open the proposal's Docs & Contract tab. In the Contracts card choose a template under Select A Contract, switch on Require signature if the customer must sign before accepting, and click Add Contract. You can attach several different templates to one proposal (each template only once — otherwise "This contract is already attached to this proposal.", though the dropdown already hides templates you've attached).

Each attached contract shows Required (toggle), Status (Pending / Signed with the signing date), a Preview eye icon, Clear Signature (for signed contracts) and Remove. A new proposal automatically gets a default contract attached: go to Catering > Settings (/admin/catering/settings) > Proposals tab > Events Contracts card to pick the Default Template and toggle Contracts Signature is Mandatory for new proposals. All contract changes are written to the Activity log.