The piece with the sharper curve (more of a point, and swoops upward just a bit more than the other) is the front.
(there's the front piece, with the sharper curve)
The one with the gentler curve (more blunted, and slightly lower than the front piece curve) is the back piece. That's because the back needs more room for the butt, and the diaper.
(theres the back piece, with the blunter curve)
Cut out two fronts and two backs.
(see the difference when front is laid over back?)
Stitch front to back (right sides together) on the outside leg edge (the longest one without a curve).
(outside leg edge stitched! Fabric opened to show you)
Stitch front to back at the inside leg edge (opposite the outside leg edge, just the straight part without a swoop or curve).
(check out the awesome decor in the background)
You should now have two front and back combinations in your hot little hands.
Turn one of those inside out. Then nestle it inside the other piece, so that the right sides are together and the seams match and all.
(obviously, don't leave your hand inside the nestled legs while sewing *wink*)
Sew these together along that curve (which is front, crotch, butt, and back seam). Reinforce the crotch area by stitching just inside your first line of stitches (i.e. slightly closer to the edge of the fabric), along the base of that curve.
Trim the edge fabric in that reinforced area, so there won't be a big bulk in the crotch (even for babies, that has got to be very unpleasant).
(trimming the crotch!)
Pull one leg out of the other, and you have an almost-done pair of pants!
Turn the pants inside out, and start on the waist casing for elastic. To do this, turn the waist fabric 3/4" over. You can either pin or iron this fabric, but I recommend not just 'eyeballing it'.
(sorry for the yellowness here, but that's the casing being sewn)
Now sew about 1/2" away from the fold, making sure that you leave a 1/2"-1" large opening for inserting elastic
Cut the waist piece of elastic. If the baby is on the small/young side, cut 15 1/2" of length. If the baby is on the larger/older side, cut 17 1/2" of length. Somewhere in between those two numbers is the length of elastic you want to have in your hands.
(measure twice, cut once!)
Using an elastic threader, or a safety pin hooked into the elastic as a 'guider', thread the elastic through the opening, through the waist casing, and out the other side. Make sure you anchor the end of the piece as you're threading, so you don't end up putting all of the elastic in the casing and having to start over. (*ahem* Been there, done that).
After you are certain that the elastic hasn't twisted while inside the casing (which is probably hasn't), match the two exposed ends of elastic. Stitch them together, 1/2" or less from the edge of the elastic ends.
(the two ends of elastic, matched and ready to be sewn together)
Stretch the waistband to pop that now-one-piece of elastic inside the casing. Stitch closed the opening in the casing.
Keeping the pants inside out, make the casing for each ankle. To do this, turn the ankle edge of fabric 3/4" over. You can either pin or iron this fabric again.
Do the same for the ankle casings as you did for the waist casing, leaving that opening to thread the elastic through.
Cut two pieces of elastic, each 7 1/2" long.
Insert the elastic, and the rest of it, the same as you did for the waist.
(the ankle on the left is done, the ankle on the right just has the casing sewn, no elastic inserted yet)
Did you sew closed the casing opening in each ankle?
Then voila! One pair of baby pants.
(Sorry that it's sideways, but you get the idea, right?)