The pattern itself is really simple and can be easily adapted to other doll sizes. You just have to know how many stitches in circumference you need (e.g. for MH dolls I use 24 st = 6 st on each needle with 2mm needles).
For a poofy underskirt/skirt, you double that stitch count twice (6 st on each needle -> 12 on each needle -> 24 on each needle = 96 in the round).
The pattern will state the stitch count for MH size, but you get the drift 😉
(e.g. for my Makie, I use size 2.5 needles in the bodice and need 32 st. in the round for her circumference (=128 st. for skirt round).
Alternatively, you can double the stitches just once, this will give a less poofy skirt part. (Only one decrease row, then)
For the skirt, I will use one size lager needles, but that is optional. If you prefer to use just one size (e.g. 2 mm for MH), ignore prompts to change needle size.
Because I enjoy your suffering, I will explain everything for knitting in the round, which is a real pain when it comes to joining the skirts. Prepare for tears and profanities (or, if you are weak, just knit the skirt part flat 😉 That will make joinning the skirts much easier, but you will have to sew up the back of the skirt part. Join the stitches in the round for the bodice part then.)
On to the pattern!