The important half of the answer above is where you do it: on the paper, before cutting. Doing it on the garment is possible and it is a worse repair every time.
If you fix it after the fact, you are taking a wedge out of a seam that already has a waistband attached and a zip in it, so you end up with a waistband that no longer matches the garment and a centre back seam with a curve in it that fights the zip. That is the version people try, hate, and conclude the adjustment does not work.
On the paper it is one fold and a redrawn seam line, and everything downstream comes out right.
Two things to watch after the wedge is folded out:
Redraw the centre back seam smooth. Folding out a wedge leaves a step or an angle at centre back. Blend it into a clean line or you will see exactly that angle in the finished garment.
Check the darts. They usually sit in the region you just shortened, and they may now be too long for the space. Shortening them by the same amount you removed keeps the point where it was relative to your body.
Once you have the number, it is yours. Write it on a card and apply it to every pattern you cut from now on — that is the actual payoff, because this is a body thing rather than a pattern thing and it will be the same on all of them.