I had seen other people wearing UFO's and got a chance to look at my friend's a bit, but since I didn't own any to even measure I had to make everything up as I went along. I do have some capri pants in a somewhat similar style, and those were definitely helpful for figuring out some of the details like where to attach straps.
These had no pattern and no trial run because they really didn't need it - I just went for it measuring and drawing directly onto some plain white fabric (a sheet to be exact). I'm pretty sure I had traced the crotch curve from a shorts pattern, but that's all. I used my hip measurement plus a couple for the waist (divided by 4 for each half leg piece) and made the pant legs 40" on the bottom (20" per leg piece). 40 inch wide pant legs are amazing by the way. They have drawstring in the waist and legs, lots of pockets, straps, and star buttons (those weren't sewn on yet in the photo above). One of the more interesting details I noticed in pants of this style is the little tucks at the knees - two on each side, so 4 per knee, only in the front. You can see them in the above photo. I believe they give you more knee room so you don't rip your pants doing crazy dance moves.
If you've been reading closely (you have been reading right?), you might have noticed I said I made these with plain white fabric. One of the scariest parts of the process was covering the whole pants in dye after spending a week figuring out all the pieces and doing all that top stitching. A couple things I had dyed before these didn't come out that great, but the dying went very well this time and I still consider these one of my most successful projects. I'd love to have another pair, but it took a whole lot of effort I could put into other things I've been meaning to make that I don't have any of yet.
I did eventually get a real pair of UFO's. They were an amazing thrift store find on a 50% off day so I got them for $2.50... quite a price break from the brand new price of around $60. But alas, they only have 22" pant legs. I did find some Ghast pants (another brand making pants for dancing) at another thrift store that I like more than the real UFO's because they have wider legs. I've also found kikwear pants, though not in my size... I'm getting a little off-topic here, but I love interesting thrift store finds like that. I never expected to find these sorts of clothing pieces.