As per my knowledge , you have to self attest the marksheet for that you have to just sign the photocopy of the document.

If you have to submit attested copy of any document then you have to know from which kind of officer you have to that with. If you need to attest it by a Grade A Gazetted Officer then you  have to visit either the DM or SDO or other A grade officers to get it signed. In that case they put a seal on the document and sign the document. This process means that the document you have producing isn't a fake document which have been verified by that authority  and declared to be true.

This is the process. I hope my answer helps you. All the very best for your future endeavors!