I haven't worked with resin, but I recently made my own doll and posted a guide-esque thing in my LJ.
http://elyneara.livejournal.com/The one made there is my 4th BJD, first one ever sculpted.
My second doll ever had some heavy modifications that I did myself as well. Doing the modding first really helped me out when it came to making one from scratch. It gave me a bit of confidence and I could gauge what I was getting into easier. Though it doesn't sound like you are looking to make a doll like mine (he's not resin) I'd like to think the guide can still be of some use. I started just because I couldn't find exactly what I wanted, and I am very picky and not willing to put up with less than what I want XD
Just FYI I have no background experience with sculpting, but I have always done art, mostly cartoons though.
Homemade doll:
I did not sculpt the head here
Modded Doll:
The head is the only modification here.
Edit: I forgot to say, when I first started I tried to work in 1/6th scale, like the obitsu 23-27 CMs and that was a mistake. I have one cat boy whose head is just terrible. Small stuff is HARD to do, I really prefer my larger attempts. So I'd say try to do a 1/4th scale for your first Full sculpt/modification.