First you will cut the sleeves off at an angle
Then turn them inside out and sew them together starting and finishing a few inches from the edges.
You might have to cut and adjust the opening for the waist depending on the size of the child.
Now there are a few ways you can finish these off.
One option is to use a crocheted yarn braid and make a draw string waist, or you and use elastic (maybe from another piece of clothing.) and hem it down so you have an elastic waist.
Below is a pair I finished using a sleeve from an XL t-shirt that was left over from making the cloth diapers.
Always be sure to save your scraps. I will continue to show you ways to use up every part.
