I usually start with my OOAK dolls the same way I start with my OOAK ponies; with tutorials!! LOL There are lots of great customization tutorials on lots of different types of sites; deviantArt, youtube, pintrest. And I often find that while I am browsing for good information on where to start I also find Tons of inspiration that guides me in the direction that I want to take my art.
Good luck with her! We'd love to see pictures as you go along!!