So to begin.
You will need: Fabric (muslin, cheap sheets, or something similar), a pencil or piece of chalk, the dress form, your body, or the body of your friend. If you are draping the back of a garment on yourself...you will either need a dress form or a trustworthy person.
You'll take your person, or your dress form, and "drape" your fabric over it. Like so.
|the green ghost|
|And if it's a piece like a bodice front, I will make the pattern so you lay it on the fold, when I make the pattern.|
I'll do another post on how to drape something with darts, or pleats/gathers later.