If you are not great at digital art, try using a vector application and break everything down into simple shapes. With vector art, you can easily move parts around and resize them without losing image quality. Maybe take a crash course on color theory too. If you use simple design, color becomes extremely important.
Vector apps: Inkscape (Win,Mac,Linux), Illustrator (Win,Mac), CorelDraw, Flash (Win,Mac), DrawIt (Mac), iDraw (Mac), SK1 (Linux), Xara Xtreme (Mac, Linux), Stencil (Linux), Scribus (Win,Mac,Linux), ZeusDraw (Mac), Karbon14 (Linux), Synfig (Win,Mac.Linux), Adobe Draw (iOS), Adobe Ideas (iOS, Android), DoInk (iOS), Serif Draw Plus (Win),Canvas X 16 (Win), Intaglio (Mac), Ink Pad Pro (iOS).