You can use the following method to create any type of round hoop, whether cone shaped, bell shaped or a crazy undulating shape. The key is that only round style hoops can be done like this; bustle/hoop combos, elliptical hoops and panniers won't work with this method. But if the silhouette of your intended hoop design is identical from the front, sides and back then this will work for you.
To start, you're going to need a largish sheet of paper, a ruler or an architect's scale, a pencil and the following measurements in inches:
your waist to floor measurement
your corseted waist measurement
desired circumference of hoop (optional; you could just draw what you like and go with the resulting circumference)
desired distance between floor and bottom edge of hoop
I've already got my figure drawn to scale, but here's how I got there-
To begin with, draw a line at the bottom of your page to indicate the floor. You'll also want to draw a vertical line down the center of your page so you can center your figure and hoops exactly. The top of your figure's head will be [your height in inches] above the floor. In my case, I'm 65" tall, so I drew a mark 65" up from the floor to indicate the top of my figure's head.
Make another mark where your waist will be using your waist to floor measurement. My waist to floor measurement is 41", so I measured 41" up from the floor and made a mark.
You can guestimate the diameter of your waist by dividing your corseted waist measurement by π (3.14). Usually your waist is more of an oval than a circle, but a corset tends to compress your waist on the sides more than front and back, making your waist very close to a circle. My corseted waist measurement is 25". Divided by π, I get a diameter of 8", so I drew a 8" wide line centered at waist level (green line in the above picture).
At this point I sketched in my figure from the waist up to that mark I made for the top of the head. I got a bit fancy and drew undergarments and whatnot, but don't worry too much about details. It's also no big deal if you can't draw people very well; the point is to just get a very rough idea of the shape of the body above the hoop.
Now you can draw the shape of your hoop skirt! So long as it starts at each end of the line you drew for your waist, finishes at each end of the line you drew for the base and is symmetrical you're golden. I made sure my shape was exactly the same on both sides by drawing it on one side, then copying it to the other side by folding the paper in half on the centerline and tracing it. Whatever shape you draw is exactly what you'll get, so take some time to get a shape that pleases you. I wanted a very bell type shape reminiscent of the early 1850's just before hoops were invented, when layering multiple petticoats gave women's skirts a shape just like an upside-down U.
First, you'll need to make a rectangle as long as the circumference of your bottom hoop (so 95" in my case) and as high as all of the measurements in between each hoop combined (in my case, 6+6+6+6.25+3.25+3.5+2.25+5= 38.25).
You can also draw in lines for each of your hoops, using the measurements you took in between each hoop to space them properly (see picture).
Add however much you want on each side for your seam allowance (yellow area in picture).
Add a bit on the bottom to fold up to make a neat hem (green area in picture). If you add enough on the bottom, you can also use it to encase your bottom hoop.
Add enough on the top to fold down to make a casing for a drawstring (pink area in picture). Ta-Da! You're ready to start sewing up your hoop skirt.
I made this pattern at TechShop. I've just posted the instructions on how to use this pattern and sew up your hoopskirt here.