This is the first stage of draping; the fabric has been pinned at the shoulders, the neckline has been cut and shaped, but the shoulderline and sideseams have yet to be pinned and given a definate shape. As you can see, I prefer to drape on half the form only. Unless you're doing something like a cross-over bodice, I do recommend just draping on half only. It's much easier to keep the pieces symetrical this way.

In this second picture, you can see that the shoulder line, armhole and sideseam have been pinned into a definable shape. I would next try this half muslin on my sister, make any noticeable changes, and make a full muslin (both halves of the dress).

I described how to drape the sleeve, but here is a picture to give you the basic idea (albiet, a rather bad one, but it's the only one I got!). You can sort of make out the pins that are defining the shape of the lower portion of the sleeve. The upper portion and cap were also pinned to the armhole of the dress. Draping a sleeve like this one is very important (if you're comfortable with draping, that is), since you need to see how the sleeve will drape when it has an arm inside it. I found in this instance, flat pattern drafting left too many 'what ifs' in the equation.