Determine to whom the check is payable. Notice which conjunction ("and" or "or") joins the account holders' names.
If the check is written to only one of the account holders, only that person must endorse the check. If both account holders' names appear on the check, joined with "or," only one needs to endorse the check. If, however, the check is payable to both account holders, with their names being joined by "and," both parties must sign the check.
Flip to the back side of the check and locate the area marked "Endorse Here."
Sign your name, or both names as determined by Step 2, in the endorsement section. Write the joint account number beneath the signatures.