The pattern I selected for Belle's gown has five large pieces in the skirt. According to the film, Belle's dress should have eight sections. This means I have to do some figuring out to turn five (or ten) segments into eight. The plan is to make the exact same skirt but with different seam lines so I can easily create the eight gathered segments.
First I measured the circumference of the bottom of the skirt, and came up with around 185". I divided that by 8 and rounded the result up to 23-1/4". Starting at the center front of the skirt, I began to mark every 23-1/4" section. The back two sections are a bit smaller (I probably mis-measured the skirt slightly), but as long as the dress is symmetrical on both sides, I'm ok with that.
Next, I pinned a ribbon from the center waistband edge to the center bottom. The back is easy, because there's already a back seam going from top to bottom. Then I pinned ribbons from the top to bottom along the three in-between points I marked on the right side of the skirt. The left and right sides are the same, so I only have to do one half of the skirt this way.
Then I took it off the dressform and ungathered the waistband. Using the ribbons as lines, I traced new pattern pieces from the mockup skirt, and then straightened them out using a yardstick. This will be a second skirt mockup to test the gathering for the top layer of the skirt. It should be exactly the same as the under skirt, but divided into eight pieces.
I serged all four edges of the overskirt pieces (so they wouldn't shred away while working with them), and sewed them into a new skirt. On this mockup overskirt, I'll determine if the overskirt pieces are long enough once they're gathered, or if I need to add more length to them.
I determined that each segment of the skirt should become 7" wide at the top (7" x 8 = 56"), since I'd lose about 1" per side with the seam allowance and gathering to make the draping. I marked this on each piece, and safety pinned the pieces together at that new waist mark. Then I turned the dress inside out on the dress form and hoop, and marked a diagonal line with pins from the new waist mark to take up the excess fabric. Essentially, I removed a triangle shape from the top of each seam. This also had the benefit of giving each piece a consistent width at the top.
Finally, after all of these strange adjustments, I gathered the seam lines on half of the dress, and adjusted the draping. This gives me the look I wanted for the skirt.