This is what I do.
Trace the size of the pattern onto interfacing or tracing paper (which ever you prefer)
Mark the placement on each piece (for this pattern it is the shorts front, back and the facing pieces) where the length needs to be added – make sure it is the same on the respective facing pieces.
Slice the pattern at the line you’ve drawn to mark length to be added.
Split the pattern to the desired length on top of another lot of tracing paper or interfacing (I like to use a combination of pins and magnets to hold it all in place).
Once you have the pieces where they need to be. Measure twice cut once remember, trace around the lengthened pattern and cut and mark this new pattern to use.
It is basically the same if you want to add to the crotch or add to the side. You have to make sure all the relevant pieces are included or you will come unstuck when sewing.
Good luck. Hope this helps.