When I was visiting my in-laws down in Georgia my Uncle Tommy gave me this cheap harbor freight pull saw that just so happened to have the exact blade thickness I needed so  I tossed the crappy handle and since it was so wobbly, I added two side rails to it for straightness and stiffness. I also mounted the side rails on the blade to coincide with the thickness of steel guide that I have been using. I mounted the guides so once resting atop the fret guide the teeth will only penetrate the fretboard at the depth i need. This makes it easy not to screw up the slots. Guide is just a piece of rectangular tool steel that I recieved  with this old Mill I had. I used to use it as a spacer for clamping and machining. I squared off the end to a perfect 90 degrees so that i can just use the long side to line it up with the fretboard and the front edge will be square. Sometimes I press a piece of wood against the side of the blade that's showing to guide and start the slot. then its a matter of how accurately you marked your slots and if you can cut exactly on that line. The pencil line width is just about the same size as the blade thickness so I just clamp the guide so that I can see the whole line and nothing more. Try not to ter up the side of the fretboard.