Some people like to start with a sketch, so they can see what they are working toward. This is a very good idea, especially if you have trouble visualizing. Then you can see what your doll will need. Do you have to alter the pattern to make the doll strike a pose? Do you need to make a prop, or find a pattern for a certain style of dress?
I have to admit that I generally do not like to draw a sketch first. Why? Because it often feels as though I am finished when I get a good sketch. Then I am no longer motivated to make the doll. I feel like I've already done the character, with all the details, and I can stop now, with this beautiful little drawing.
I need to feel motivated, and I enjoy the challenge of working directly in the cloth. I will make an arm over and over and over again to get it to come off the shoulder at the exact right angle that I want. I had piles of discarded limbs from several different dolls to prove it! I would keep them around for years, hoping to reuse them at some point. But it was rarely the case.
When I did "Decisions, Decisions" I remade her arms several times. It was important that they look graceful and natural. And when your arms are raised, they are supported by muscles that come up from the front and back of the torso. You can simply move the arm pattern (the way a Barbi doll's arm moves in its socket) but it doesn't give you the realistic affect I was looking for.