how to use previous year capital loss to deduct tax for current year

Claim Net Capital Losses. If you want to use net capital losses fromprevious tax years to lower your capital gains in the current tax year, claim a tax deduction on line 253 of your tax return.

Double click line 253 on T1 form.

What happens if you have a capital loss? 

If you have a capital loss in 2015, you can use it to reduce any capital gains you had in the year, to a balance of zero. If your capital losses are more than your capital gains, you may have a net capital loss for the year. Generally, you can apply your net capital losses to taxable capital gains of the three preceding years and to taxable capital gains of any future years. 

Find capital gain & loss carryover from CRA. Use net gain || lose for line 253 on T1.

Capital loss carry back

Fill T1A form

Apply to previous 3 years.