The other day I was planning to cross stitch an image for my phone cover. But I couldn't find a picture I liked, and I already had the clear case. And a stack of washi tape (love this stuff!) Enter "aha moment!"
(I wouldn't call this a tutorial, because I'm pretty sure my toddler could do this, but just in case:) I cleaned my phone, then applied the washi tape directly to the outside. (It's paper tape, and has a pretty low sticky-residue-leaving factor.) Then popped on the case, trimmed the edges with an exact knife--being very careful not to cut into my phone--and voila!