Nicky Epstein has several books of Barbie patterns, (barbie patterns exclusively, and barbie and me (matching outfits for barbie and babe’s (childrens, teens, adults!)
if you’re on ravelry, there are barbie knitters (as a group) and many links to free patterns (you’re bound to find one you like/for the yarn (weight) you want to use)
as for designing… well that’s a whole different skill, but basicly
swatch, swatch, test knit, (record what you are doing as you do it, (frog --or reknit) record–again,
have some one test knit what you recorded, edit, test knit again.