Old, adult sized tees
Pair of leggings that fit your daughter
Large piece of paper to trace a pattern
First thing you need to do is make a pattern. Take your leggings, fold them in half and line up the straight edge of the leg with the edge of your paper. Then trace around them leaving about 1.5 inches on the top and 1/2 inch around the back and then the bottom of the pattern needs to stop about 2.5-3 inches from the crotch(...really?? is there no better word??) depending on how long your want the shorts.
Cut out your pattern and mark the straight edge 'Cut on Fold'
Now it's time to use the pattern. Lay your tee shirt down as flat as possible. Place the straight edge of your pattern along the side seam of the tee shirt, aligning the bottom of the pattern with the bottom of the tee. The tee-shirt hem will be the hem of your shorts. Trace around your pattern with the disappearing pen. Then flip your pattern upside down and do the same on the other side seam of the shirt.
Cut out your pieces, open them up and lie them down right sides together. Pin and sew from the top to the tip of the crotch on each side.
Then, refold your shorts so that the two seams your just sewed are now down the center. Pin and sew the crotch closed, from one leg to the other. Almost done!!
Now you just need to make the waistline. Easy peasy. Just fold the top down a 1/2" then 1" (approx.), and sew it down leaving a 1" hole for inserting the elastic. I also often sew around the top of my elastic casing and find that it prevents the elastic from turning and twisting. I used 18" of elastic for my nearly 4-year-old.
Once you've inserted your elastic, overlap the ends and sew them together. Then sew your hole shut. Flip those babies right side out, maybe embellish them a bit and you're done!