I am not an expert, but here are some suggestions based on my efforts to use an inkjet printer with waterslide decals. others with more suggestions should chime in, but for starters:
if you are printing onto a dark background you need to print on white waterslide decals. printing with clear decals only works if you have a very light color on your pedal.
use vivid colors for your best contrast between the decal and pedal color.
after you print the decal and it dries, you should spray a couple of clear protective sealant coats on the decals before putting them in water or your printed colors will run.
print your test runs onto plain paper until your layout is right.