Understanding how the "One Size" sizing system works is quite easy as the system works in 3 general size categories. The 3 categories are:
One Size Regular- Fits sizes 0 - 10
Plus Size- Fits Sizes 14 - 22
Extra Plus Size- Fits Sizes 22 - 30
NOTE 1: Size 12 is a transition size and it really depends on body type. A longer athletic style body should probably still fit into the One Size Regular and a more booty heavy body type should be going into the Plus Size. The same would be for size 22 and 24. Also, keep in mind how tight you want your fit. If you are at the end of the sizing guidelines, it will be a tighter fit. If you prefer a more giving fit, go into the Plus Size.
IT IS IMPORTANTthat you are honest with your sizing. If you try and fit into a very tight fitting One Size you will sooner than later tear the seams. Leggings are literally held together by thread so if you are too big for the One Size, they will tear. Be honest with your frame and body type.