For the affidavit, mention your name, age, the name of parent or guardian, place of residence, permanent address, correspondence address, details of exams cleared, details of the gap year, the period of the gap year, reason and purpose for the gap year. 

Along with this you also have to declare that you were not doing anything illegal during the gap year. Also, mention that whatever you have written is true to your knowledge and is 100% authentic.

After you have done this, take a print out of this on a stamp paper. You will then have to get it notarized by a notary officer in your area. 

An affidavit is a legal document, so the last step is mandatory.