Sign In with the wallet specified in the PoE or Sale attestation as the counterpart to the signature.

Click on the wallet dropdown (it is where the Sign button was before connecting your wallet) and choose Signatures in the dropdown

Tap the Pending option in the tab menu. A list of all your pending signatures will display. Find the attestation to countersign.

Click on the "Countersign" button or tap the three dots all the way to the right of the entry’s row and choose Countersign.