Shadow Hunter, You don't sand the sides. You use a sanding block/sand paper and sand both back and belly of the limbs. Sand the working part of the limb, from the fadeout to the last couple of inches. You must sand both sides, and sand both limbs. You want both limbs to be "balanced". Measure the distance from string to the top of riser, then measure the distance from the string to the bottom of the riser. If they are 1/4 inch difference, keep it that way. Bottom limb should be a tad stronger, and will have less distance from bottom of riser to string.
You will be surprised how much sanding it will take. Glass is tough.
I have only done a couple bows, so I am not an expurt, and this advice is worth what you paid for it. But that is what you need to do to weaken the limbs.
Dan