How to make a gymnatic outfit for your american girl doll out of a baby onsie

STEP ONE: Carefully cut your onesie apart at the seams. You will end up with four peices -- front, back, and two sleeves.
STEP TWO: Lay your front and back pieces over your doll, right sides facing the doll, starting with the back and then the front. (This way they will be on the correct sides when you turn it right side out.)

Pin at the shoulder seams. Very gently pull it over your doll's head. This is a good way to be sure it will come off and on!
 STEP THREE: Sew the shoulder seams slightly to the outside of your pins. You will only need to sew from the edge of the ribbing on the front to the edge of the ribbing on the back.
STEP FOUR: Decide whether you want short sleeves or long sleeves. (I chose to do long sleeves this time.) Place a sleeve on the doll and mark at the top of the shoulder where the shoulder seam should be.
Cut the sleeve all the way across at that length, making sure to add a seam allowance. (I didn't cut mine very straight!!)STEP FIVE: Determine how wide you want the sleeve. Mark each side with pins...
...and cut a sleeve shape. Don't forget the seam allowance! (I did; the sleeves ended up a little too tight!)
I put the onesie back over my doll's head, inside-out as usual, and marked with pins where the underarm seam would be. I then cut away some of the excess fabric, leaving a seam allowance, as shown in this photo.
STEP SEVEN: Attach the sleeve! Pin the sleeve in half and mark the center. Line it up with the center meeting point of the front and back of the onesie. Lay them right sides together, matching the pins, and pin the sleeve on. You may need to stretch the sleeve piece a little to make it fit the arm hole.
STEP EIGHT: Go ahead and sew the underarm seam of the sleeve, stopping at the armpit.
STEP NINE: Back on the doll again, inside out! Pin the side seams down past where the leg holes will be -- approximately mid-thigh.
Sew!
STEP TEN: Back on the doll again, inside out, then AFTER checking to make sure the fit works, trim away the excess fabric.
NOTE: If you like, you can stop here and make a little gown! Cut the length down past Dolly's toes, make a quick little casing, thread some narrow elastic through, and you are done! I've made a few of these, and they are just so cute 
STEP TEN: Trim one side of the front to create a onesie shape. You do not need to leave a seam allowance.
 Remove the onesie and fold it so the side seams are together, then trim the other side of the front to match. You can then trim the back approximately the same, leaving it full length in the back.
STEP ELEVEN: On the doll AGAIN. :o) Trim the back to a more appropriate length. Remember that the back will need to overlap the front.
STEP TWELVE:Using the smaller of your two leftover onesie scraps, cut two strips, approximately 2" wide. Sew them together to create one long strip.

STEP THIRTEEN: Open one of the side seams a couple inches.
Begin sewing your strip to the lower raw edge of your onesie, right sides together. Give your strip a little stretch as you go. You'll have to fiddle it around the curves.
You will sew all the way around the lower edge. Then re-sew that side seam you opened up.
sew the open part just like the onsie was
 and you are done

Extra:You can glitterize your outfit with some glitter glue or sparkles!!