Not pretending to be an expert, but the couple times I've helped people fit up a corset I've found it useful to start with brown paper bag pieces because they don't flop about so much, or muslin. 

I need to make a new corset (for use with 1810-1825 - “up and out” but not saggy bosom looks) so here's what I'm going to try this time. Any comments appreciated: 

Make a tube out of two pieces of muslin. Both pieces should be long enough to go from collar bone to mid- or low-hip. One piece should be wide enough to go from side seam to side seam around the back. One piece should be wide enough to go from side seam to side seam around the front. Make sure it is wide enough to skim the largest circumference (hips, tummy, or bust). Add a bit so that it is loose, not confining, around the bust. Mark the center of each width. See up the side seams so you now have a tube. 

Put that on so that the side seams are at the side. Tie a string snugly around the underbust. Make sure the center marks are centered. From the front center, finger pleat the fabric under the string so that is evenly distributed. Not for the graphic part: hoist the girls up and out. Smooth out the fabric from the side seam to the edge of each breast (you want to keep all the excess fabric under the bust, with smoothness around the sides and back). (Here’s a tricky but - I’m wider on top than bottom. For those with the reverse issue, you will have to arrange the fabric so you can have suffihip gussets.) Now get out the duct tape and cut off a few hoist and separate again