In order to cut fret slots by hand, you’ll need a sharp knife, a straight edge, and Level. First, remove any excess wood around the fretboard, using a sharp knife. Next, use the straight edge to mark the position of the first fret on the board. Make sure the line is perpendicular to the board and level. With the knife held at a 45-degree angle to the board and your finger on the back of the blade, carefully cut through the wood above the line.
Repeat this process for each fret on the board. Be sure to use caution when cutting; a sharp knife can easily slice through your fingers. Once all of the slots have been cut, sand them smooth with a sanding block or gritbrush. Finally, paint or stain your new fretboard assembly according to your desired finish.