Right after I had my baby my friend gave me a couple of button up shirts for those days when you are constantly breastfeeding and don't fit into your normal clothes yet. This is one of those shirts, and although it no longer fits me I always loved the fabric. So I thought this would be a perfect excuse for another up-cycle for my daughter.
This tutorial will work with almost any women's shirt - not just button-ups, but the fabric should have a bit of drape to it. My daughter wears a 4T usually in ready to wear sizes so the measurements are for that size, although you could easily size it up or down for your needs. Because of the elastic neckline the sizing and measurements are pretty forgiving.
Here are the approximate measurements that I used:
First, finish the long edge of the sleeves with a 1/4" hem. (for most of my seams I am simply serging and folding, although feel free to do any seam finish that works for you).
With right sides together, stitch front and back together at side seams at 1/4".
With right sides together, sew the sides of the sleeves to the top of the back and front at 1/4".
Turn under the seam allowance of the remaining armhole and stitch at 1/4".
Fold the neckline to the inside at 1/2". Stitch at 3/8" leaving an opening for the elastic (but stitching the button-closure closed).
Thread a 20" piece of 1/4" elastic through casing. Now adjust the elastic to the desired length, trim, sew ends together, and then sew up opening in casing.
Clip threads, iron, and you are done!