Usually I would baste the top of the sleeve and then pull the thread to gather it at the top, but instead I just fold it over into pleats and pin them as I go.
First you cut out the two facing pieces and then sew them together to end up with this.
Then you pin it on top of the dress with the dress right side out so that you have right sides to right sides and sew slowly along the entire neckline.
Once it's sewn, flip it to the inside and then sew it again so it stays put. I always pin everything but you don't have to.
Because it's a little short, I cut out 2 more long pieces of fabric to make a ruffle on the bottom. I did a double hem like I did on the sleeve before pinning it in pleats onto the bottom on the dress and sewing it on.
Here is the finished product: