one of the best ways is to copy a pair of yoga long pants as it's a stretchy knit fabric.
you can ask to borrow someone's pair.

on a large sheet of paper
fold yoga pants in half.
the front pant leg will be slightly narrower than the back leg.

place front pant leg on sheet and copy.
do same for back.
the back crotch seam should have a deeper arch than the front crotch.
add seam allowances on each seam line 5/8".
if you are bigger than the pattern, add more inches on the sides of the leg seam, outside seam, Not inner leg seam.


cut 2 fronts and 2 backs.

if you don't have a stretch stitch on your machine. use a narrow zig-zag stitch
a straight stitch will break if you stretch the pants while wearing it, esp on the crotch seam.