gunrunnerjohn posted:I print the numbers in reverse colors, white on black using Microsoft Word. I use canopy glue (Tester's Clear Parts Cement) to glue them to a blank plastic piece cut to size. Touch up the edges with a Sharpie so no white shows through and place them. Whenever possible, I have the numbers on the inside to protect them.
For the removable number boards on headlights, I sand/polish the old numbers off and glue the above created numbers to the plastic. A couple coats of clear coat will seal it as those number sheets are, by necessity, on the outside.
Really that's the best way. Some do not have M.S. skills. Just about all signs can be made on the PC if you know the workings of Microsoft Office items. 
