Be sure that you only cut one side of the stitch. Sometimes people just grab the knot and cut it off. This leaves the suture in the skin. If the doc was nice they left little tails of suture on the knot. Grab the knot or the tails, lift it, get your clean cuticle scissors flush with the skin and cut only one side of the suture loop. The loop of suture will then slide out as you pull on the knot. Quite straight forward in most cases.