I have invented my own for kids. and have made myself a skirt with shaping at the hip line and a zipper.
You need to choose your yarn, figure out your gauge...decide what style and have at it. You can make them just like sweats with an elastic waist....in which case you just cast on enough for a relazed fit at the hips and knit a ribbing band which will be turned down for elastic, work until you come to the crotch part....as long as you wish....use another pair of pants for a guide....then divite in half and knit the legs....tapering or not as you choose.....when the legs are long enough....make a "turning row"...a row of purl from the knit side...knit another inch....bind off and hem the bottoms.
Possibilities for things like a cable down the sides or very tapered cullf or widening cuffs or a rear pocket are all there.
Linda C