To make these pants, you will need:
a long sleeve shirt (or two long sleeves of the exact same size)
elastic 3/4" or 1", as long as your babe is around
pins, if you like, I don't use them usually
a big safety pin
a pair of pants that fit properly around the waist and diaper (to use as a guide)
Get Your Pieces Ready
Make sure there is about two inches of fabric to spare above the waist of the guide pants- you'll need it to make the casing for the elastic.
Once you're all lined up, cut the sleeve along the seam that goes from the crotch to the waist of the guide pants and continue up in a straight line once you reach the waist.
Leaving the guide pants where they are, cut the sleeve approx two inches above the waist of the guide pants. Better to have more room for the diaper than less...
Make the same two cuts to the second sleeve as you did to the first, using the first as your guide.
Give Those Pants Some Legs to Stand on!
Stuff it inside the other sleeve, lining up the cuffs and the existing seam.
Take this little sleeve bundle to your sewing machine and sew along the U-shaped line that will make the crotch of your new pants. I like to start at the center and sew to the waist, so I can make sure the two existing seams line up perfectly.
Haste Makes Waist (wait, what?)
Fasten your big safety pin to one end of the elastic and insert it into the casement.
Keeping the elastic from twisting, scooch (yes, that's a technical term) the pin through the waistband, pulling a bit of the elastic in each time. Don't pull the elastic all the way in though, we still need to sew the elastic to itself.
When you have finished, your pants should look like the picture on the right, with elastic coming out each side of your gap in the waistband casement. Remove the safety pin.
Then spread the seam allowance and sew it down too.
Unbunch the waistband until all the elastic is in the pants and the fabric is uniformly gathered or ungathered.