Here's how I refashioned a t-shirt into a skirt for my baby girl, Macy.
I used a medium shirt from H&M and it fit my 6 mo. old.
Cut out the bottom piece of the shirt about 8 inches tall for the skirt. This works great because the bottom hem is already there! Then cut out another section on top about 6 inches tall for the waistband. You only need the front of the shirt for the band.
Then, gather the top of the skirt. To do this, sew a simple long stitch all the way around. Then pull the thread tails till it gathers to about the width of the waistband.
For the waistband, fold it over long ways and then sew the ends inside out.
I stuff the waistband inside the skirt and pin it from side to side making sure the right sides out are facing each other on the inside and the seams are lined up on the sides. Then sew it, flip it right side out, and you've got a cute baby skirt.