After you file

Well...in a way.

 

The per-diem $$ you received will probably be included in your W-2 $$.

You just enter your W-2 data and it's taken care of. 

Yeah, any per-diem paid to a W-2 employee ends up being considered as added taxable income.

____________*Answers are correct to the best of my knowledge when posted, but should not be considered to be legal or official tax advice.*