If you didn't report that trade or trades on your 2015 income tax return then the answer to your "How do I do this" is "you don't."  You'd need to amend your 2015 income tax return to report that loss.

If you did report that loss in 2015 then if the loss wasn't fully used up by offsetting capital gains that year some amount - $1,500/$3,000 depending on filing status - was offset against "ordinary" income, like wages, in 2015 and if there was still loss available that amount would be carried forward to 2016.  That loss first would be offset by capital gains.  If there was still loss than once again $1,500 or $3,000 would be offset against "ordinary" income.

This is a purely mechanical process dictated by the tax laws and beyond that you can't affect how the loss gets used.

Tom Young