BAH! My internet went out last night while I was trying to answer your question
I did, however, save my entire write-up, so here ya go:
More or less, that's how it works. Never having done this myself, here's my thoughts:
Now, before you cut out the pattern pieces, make sure the size you've chosen will actually fit properly by measuring at the waist and hip lines and subracting the seam allowances and find the waist and hip sizes that are, in total, only about 1 inch bigger than your actual measurements, which is called the ease. Most pattern companies, for some demented reason, draft 3 inches of ease into their patterns. If the waist and hips are two different sizes, no problem! Just freehand a nice curve to join the two different sizes.
With that out of the way, you can copy all your pattern pieces onto packing paper, wrapping paper, or any other large paper, or tape lots of waxed paper together. That way you won't be cutting up your original. I like waxed paper because it's translucent, so you can trace your pattern really easily. For the actual bellbottom part, pick an angle for your flare. Say, 35 degrees. You want the flare to be symmetrical on both the front and back pieces and on both the inseam and outseam, so a protractor is your friend, but not absolutely essential if you can eyeball it close enough. So now figure out how far up from the hem you want your flare to start and mark that on both the front and back pieces. Then draw the flare lines at your desired angle. Mark a new hem at the end of the flare lines that is the same distance as the original hem. Now you're going to have to draw a fancy shmancy curve all the way from the end of one flare line, down around the original hem and out to the other flare line. A pin, string and pencil are useful for that. Just stick the pin halfway between the tops of the flare lines and use the string and pencil as a compass to draw the hem. Once that's done for both the front and back pieces, cut out your new and improved pattern and off you go!
Cuz some people work better with diagrams, TADAA!