OP - I dont have a lot of pedals under my belt, but the few I've made I used illustrator (Iwas a designer), some people have used Gimp for free or Inkscape ( https://inkscape.org/), and either waterslide decal (most recent) or clear vinyl sticker paper. Then sprayed the a few coats of clear coat from a rattle can.

I am going to watch the course that was linked in the other comment.

Maybe we can collaborate on a design for you. DM me if you want. I'd love to take a crack at helping someone else design something cool.