Andy (Crimea River) might be able to advise about masks, Steph, as he recently got a plotter / cutter.
Decals can be made fairly simply, although the definition depends on the quality of the printer, if doing it from a PC, with a laser printer being better.
However, white can not be printed, unless you have access to an ALPS printer, so white numbers with a coloured outline, for example, can't be done easily.
The decal kits available from such as Testors, are the same materials commercially available, at much lower prices, from computer paper suppliers, and are available in clear sheet and white sheet, for both inkjet and laser printers.
Some designs can be printed onto the white sheet of course, and exhibit much better density than those on clear sheet.
Printing, for example, code letters in a 'solid' colour, other than white, onto clear sheet works reasonably well, although again the density is not great, due to the nature of the printer, and the clear background. Black letters aren't too bad, but reds, blues and yellows, for example,lose density when transferred to the model background.
I've made Luftwaffe unit badges on both clear decal paper, and on white, where the design allowed the use of the latter and, on the clear versions, i have used this as the basis, and, once applied to the model, I have then 'filled in' the less dense areas with paint.
I also had a similar problem that you have, when needing a white letter or numeral, with a coloured outline. In this instance, I printed the decal as an outline number, on clear sheet, the outline being black, and applied it to the model, 'filling in' the outline with white paint, as seen on the 1/32nd scale Bf109E below.
The white 'Tatzelwurm' on the nose was also done this way, and the fuselage crosses were home-made decals, scanned from those from another kit.