Then you just write! Plan out your design (a bit better than I did here) & draw. I may retry this DIY in the future now that I know these paint pens rock! I think this one & this one would be some cool inspiration. I didn't try this because I don't know anything of sealers...but if you could find a sealant that wouldn't mess up the plastic then you may want to put a coat on here, just to be safe.
I've pinned a whole load of cool Paint Pen inspiration to my Handmade Inspiration board. I can't wait to try them out!