To establish whether you need to carry out this alteration we can employ the elastic trick we used in this post to establish where the natural waistline actually is. Put the elastic on over the toile, let it settle at the natural waistline, and measure the distance between it and the waistline that is marked on the toile. This will tell you whether you need to lengthen/shorten the toile and by how much.
"If you are short waisted you will need to shorten the pattern above the waist. Draw a line from the centre back to the side seam, 5cm (2in) above the waist. Fold along this line and make a pleat to remove the excess body length; pin and then stick in place. Re-shape the seams and then repeat the process on the front pattern"
The image on the right is the one from the book that corresponds to the text above.
I'd personally be inclined to cut and overlap the pieces at the waistline rather than fold out the excess as I find it a bit fiddly to fold with as much neatness and accuracy as I can cut.
I would add that any shortening above the waistline will of course shorten the final length too so it may be necessary to add back on at the hemline, the amount removed above the waist.
To reshape the seams simply trim off any resulting "jutty out" bits using the original side/CB/CF lines as a guide.
" A long waisted woman will need to draw a similar line 5cm (2in) above the waist and cut along this line. Place a piece of paper underneath the pattern then spread both sections of the pattern equally to obtain the required body length. Pin and stick to the underpaper, re-shape the seams and then repeat with the front pattern."
I'd be inclined to mark the "underpaper" first with two parallell lines the required distance apart, and stick the pattern pieces down along theses lines. This will ensure that the length is being added in evenly and accurately across the waistline and that the side seams will line up again afterwards.
As with shortening, lengthening above the waist will ultimately make the top longer at the hem too so you may need to remove from the hem, the amout you add in at the waist.