In order to sign a transaction a user needs to be part of the required signers of the multisig to which the transaction belongs. On the transaction page, you will see details for the transaction to be signed. After reviewing, users can sign by clicking on the "Sign" button. This will open the Cardano wallet interface prompting the user to sign the transaction.

If the signature is valid, a toaster notification will show up indicating that the signature was applied. The corresponding signature will turn green after a few seconds on the signers card.