Most of Rowan's clothes are hand-me-downs or consignment/thrift store buys. Because I'm thrifty. And because she grows out of everything so fast, what's the point? One BIG issue is the shift from season to season. We bought a 4-pack of beautiful long-sleeved onesies (babygrows) from a Carter's outlet ($5 for 4!) ... that she hardly wore because they were too big. Since most of them were in good condition, I combined them and turned them into summer swing tops. I want them to last a while, so just cutting off the bottom and sleeves was not enough. My goal was to make them wider too. That way, when they start getting a little short, it won't matter as much. This is not a great tutorial I'm afraid. My "demo" onesie ended up a mess because I got distracted mid-project. But here's what I can show you.
Lay out the onesie and cut the bottom off. Cut the sleeves off at an angle. This gives you maximum length from the sleeve fabric and creates a nice cap sleeve for your top.
Cut the sleeve open and lay it next to the body. You should have more sleeve length than you need - this is good. It gives you some wiggle room.
Pin and stitch the sides first. Don't worry about having extra fabric at the top. Once the sides are closed, you can play with shaping the top.
If you insert a rectangle, then flip the top right-side out and you'll see that it bulges at the top of the insert. And begs to be folded down and top stitched. Easy. I am not a sewer. I'm a hacker and I pulled this off. To finish, zig zag a couple of times around the hem and the sleeves. Go ahead and pull on the fabric ... the stretching will help it ruffle.