How do I apply capital losses correctly? Why did not refund not change after applying them? How much capital loss should I apply to the previous tax year?

If you have incurred a Capital Loss in the current tax year, you will not see a change in your refund amount for the current year. You can apply the current year Net Capital Loss to any Capital Gains you incurred in the three previous tax years as a Net Capital Loss CarryBack, (or carry the amount forward to apply to future Capital Gains).

You do not have to complete a separate form Loss Carry Back with CRA, Turbo Tax does that for you. You do have to enter the amount of the loss you are applying to each year and that will allow CRA to adjust those tax returns.

As a Net-Capital Loss can only be applied against prior years Capital Gains, you'll need to review those tax returns for income at Line 127 to determine if and how much to carry back.