At some point, you may want to reduce the waist measurement of you pattern.
If it is by a small amount (up to 6cm) you can just remove it from the side seams (this tutorial will show you how), but if it is more than that, you should also consider adding waist darts to your pattern (which you can later transform into panel lines, if you would prefer not to have darts).
In most cases, it is best to distribute the amount you are removing by combining the two techniques (adding darts and removing some from the side seam).
I have used a bodice pattern for the example, but this technique will also work for a skirt for trouser pattern.
To start, take your front and back pattern pieces.
Work out the amount you will be removing from the waist, and divide the result by 4 (as you will distributing the amount you are removing from both side seams, of both the front and back pieces - i.e. four different pattern pieces).
For example, if you would like to remove 4cm in total, you need to remove 2cm from each side seam (1cm from the front and 1cm the back)
Take your result (that has been quartered) and mark a point, on the hemline, this far in from the side seam.
Mark the point on both the front and back patterns.
Draw a line, connecting the points you just marked, with the underarm point, on each pattern.
This is now your side seam.
As you can see, only the waistline is affected. There has been no change to the bust measurement, or armhole.
Cut along your new side seams to finish.