I worked for a employer who never paid me $300 on last pay check and not even put it W2. How do I report to IRS that I have unpaid wages lost?

You don't report it. The IRS is only interested in money actual paid and received and/or reported on a form (e.g. W-2).

You my not deduct money that you were due  but not paid. Since you don't have to report it, you have effectively already deducted it.