Having your paint very thick helps, as does painting on an area that is dry or almost dry. You need to hold the guide firmly against the surface as you are using it. If the surface of your painting has texture then it's hard to keep some paint from sneaking under the edge guide.
If you want a truly perfect crisp edge, use tape (Google “Frog tape”). After you put the tape down, put a layer of matte or gloss medium along the edges and let it dry - that solves the issue of paint bleeding under the tape.