1) Cut the fabric (front, back, leg bands (2), waist band, crotch (2)). I used two different colours to ease demonstration.
2) Pin the back of both of the crotch piece to the back piece, with the back piece sandwiched between the two crotch pieces.
3)Pin the front of one of crotch pieces to the front piece. Roll up the front and back pieces so they sit against the back crotch pinned area/seam. (Note: it may be easier to sew the back seam so nothing slips. By this point I had sewn a few so I was combining steps.)
|(Front and back pieces rolled up between crotch pieces.)|
|(Front and back pieces sandwiched between crotch pieces.)|
|Front and back pieces still rolled between (now serged) crotch pieces.)|
|(Seams are now hidden between the two crotch pieces.)|
|(Seams are now hidden between the two crotch pieces, both sides.)|
|(Leg band stitched to one leg opening.)|
9) Attach the second leg band to the second leg opening. (You could do both leg openings at once, but don't close both side seams.)
|(Attached leg bands with one side sewn.)|
I ended up top-stitching around the serged seams to re-inforce and to have the seams lay flat.