One of the easiest ways is with a 'drawing' softare package on your PC. For example, Adobe, Corel, Serif etc.
You can either produce what you want, or scan, say a 1/48th scale existing decal sheet, then re-scale it and print it off. Depending which system you use would govern the way to re-scale, but, for example, if you know the size of the original, it can, theoretically, be re-scaled to any size.
If you have a 1/48th scale letter or character that measures, on the decal sheet, one inch, then the original 'full size' would obviously measure 48 inches. So, in say 1/8th scale, that should measure 6 inches. Therefore, take the original design measurement (1 inch) and re-size, using the software's system, to the required size (6 inches).
You should find, even in a basic package, or in the PC's own font's, that you should be able to design and draw most types of letters, numbers and shapes, even with surrounds, inline/outline, colours etc., without too much trouble.
It should work OK in the scales you are probably talking about, in the smaller scales of plastic modelling, it is sometimes a little too 'soft' around the edges, due to the printing process.
Hope this helps,
Terry.