What form is used to carry forward losses?

Assuming you are talking about net capital losses, the carried forward losses are entered on Schedule D in the immediately following year. There is no special form.

BUT be sure to fill out the carryover worksheet in the Schedule D instructions before putting down an amount on Schedule D. For low income taxpayers, the amount to carry over is not the obvious amount. Many people cheat themselves out of part of their carryover by not filling out the worksheet first.