In a previous tutorial, I detailed how to shorten a pattern (using a sleeve as an example). That method of shortening is necessary when you do not want to make changes to the hemline (shape and width).
If you want to simply "cut-off" a pattern as a means of shortening, then this tutorial details how you should go about it.
Take the pattern you will be shortening. In the example, I am shortening a shirt pattern that extends to high-hip, by cutting off at the waist.
On the CENTRE FRONT, mark the point where you want your pattern to finish.
From the point on the CENTRE FRONT, draw a perpendicular line through the pattern, that intersects the side seam.
To transfer this line onto the back pattern, lay your patterns side by side, so that the side seams are matching.
Mark the new line onto the back pattern piece, at the side seam.
Draw a line that is square with the CENTRE BACK, and intersects with the point on the side seam, to create your new hemline (or waistline in this case).
You can now cut along the new hemlines, to remove the excess length.
To complete the pattern you will need to check that the new line runs smoothly at the side seam, before adding seam allowance.