Have you sewn before?
You may find that purchasing a pattern will be the most detailed way to figure it out, if you're new. A pajama pattern would have basic elastic-waist shorts and pants and might be something you'd use again ....?
If you've sewn before, then this is what I do - I just trace around a pair that fits the way I want them to, cut them out (adding in seam allowance and casing fabrics space), then whip them up like sewing any other pair of pants (center seams first, then the side seams, then the casing, insert the elastic, then hem them).
Not all who wander are lost.