I am not going to teach anyone here how to draft a corset pattern, so I am doing this part of it assuming you have basic drafting skills.
The way I make a corset with cups in it starts out with your basic SLOPER. A SLOPER in case you don't know is the pattern of the basic body shape of a standard size (or custom to your size) with no seam allowances allowed. You can find sloper patterns online or if you have a college with a fashion program in your area it's usually pretty easy to talk them into letting you copy off a sloper pattern or two.
Here is one of my old beat-up ones for reference. I will be using a basic size 6 sloper with a B cup size for this tutorial, you may of course size yours to whatever you need it to be.
Basic Sloper Pattern Front and Back
As you can see the front part has a "circle" in it. This circle is the most important part for drafting your cup corset. The "Circle" is a paper representation of the full breast. With the point in the middle being the nipple.
Close-up of bust part of Sloper
The lines running throughout the sloper represent the fit lines. If you took all these lines in you would have a mold of a body shape so to speak. This is what we will essentially be doing since a corset is a tight fitted garment.
I am not going to show how to do the back part of the cupped corset since the cups have nothing to do with that part. Again I am not going over a basic corset draft. I am also not going to show how to pattern in the hips parts of the corset as I assume you all have your own methods for that. So think of this as just the upper half of the corset or a Bustier that stops at the waist.
Next you will need to copy off your sloper onto paper. Make sure and draw in ALL THE LINES as you will need these for fitting.
Drawn off Sloper. This is a crude quick drawing, done for illustration only, you should actually make your circle round, not all hand drawn and wonky like mine :)
Next you need to block in the basic lines/shape you want your corset to be. This includes the "seam" lines.
Basic blocking of shapes, shown in red.
Remember all those "fit" lines you copied on there. Now is the time to cut along each of them. There is a straight line and an "angeled" line on each fit line. You will need to cut the straight line NOT the angeled one (the straight line allows for some ease) on ALL THE LINES EXCEPT THE BOTTOM TWO. The bottom lines are the darts. Since this is a corset you want the tightest fit on anything but the bust so cut the angeled lines out of the bottom dart.
You will end up with something like this.
Next you will need to close your lines. Use tape to do this. I have used leopard print tape so you can better see what I did. Close your lines only up to any point involving your corsets shape. Your shape will now be off and wonky this is OK you will fix this in the next step. When all "darts" are taped close you will have something that looks like this:
Notice the design now looks like a cone. I have shown it above from the side as well so you can see this better. Basically if you put this on now it would mold over the bust shape.
Next you need to "true-up" aka straighten out your design lines. I always add a bit more to the height of the cup to make sure nothing will spill over.
You need to decide where the seam in your cup will be. I mark the middle of the cup "the nipple point" with a dot, on this design I have made my seam where it will match up with on of the corset seams, for a nice flow together. You can make your seam in the cup wherever esthetically pleases you most.
Next you need to cut your corset pattern out on the design lines. Make sure and mark all the pieces, especially the TOP AND BOTTOM of the cup. I use the letters T and B for this.
Lastly you need to trace all the pieces off on paper. Smooth out any wonky lines. If the cups have pointy bits at the nipple points when you cut them apart be sure and curve the line or you will have a cup that fits ala Madonna cone bra style. I also mark the nipple points on each cup piece so I can make sure and match them up right when sewing them together later on. Use whatever seam allowances you like. I use 1/4 inch on the cups and 1/2 inch on the corset side seams.
And that's it as far as making the cup part. Next you would pattern the back. Draw in your hip shapes/lines, then re-draw the whole finished corset pattern and get ready to sew.
If you want to pattern cups that do not have a gap in the middle of the two for a more "push-up/push together" boob effect just make the bottom curve of your cup closer to the center front of the pattern.
I'm off to go out of town now. When I get back I will post a sewing the cups into a corset and various ways to finish them (underwire, heavy lining, straps etc) tutorial part 2.
For quick reference here is one of our finished designs using this method of drafting a cupped corset (and yes the models boobies didn't fill it out which is why the cups don't look filled in):