The signature line is in the bottom right corner of the check. This is the last field you fill in — an unsigned check isn't valid, and a bank can decline to process one.
What counts as a valid signature
Your signature should reasonably match the signature your bank has on file for the account. It doesn't have to be pixel-identical, but a signature that looks nothing like your usual one can get a check flagged, particularly for larger amounts.
For business checks, the signer needs to be someone authorized on the account — banks keep signature cards on file for exactly this reason.