Here's my method: admittedly not the best, but I make do with what materials and machines I have.
Lay out the artwork right-reading, full-size (I use Corel Draw). Print either on self-adhesive label stock or plain paper (I use a laser printer).
If plain paper, I use 3M spray adhesive to stick the paper to the panel.
In either case, when the artwork is adhered to the panel, I cover it with a clear plastic laminating sheet.
You can also print directly onto the laminating sheet with a laser printer, but this isn't as permanent, as the toner can eventually wear off. Covering it with clear plastic makes it pretty indestructible.