It has been really hard to find long sleeve onesies for my littlest guy. He has lots of outfits with long sleeves and hoods, but I don't like putting him down for naps in a hooded outfit. I transformed some of his short sleeved onesies into long sleeved ones.
I started by measuring one of his existing long sleeved outfits. (He's in 9 or 12 month sized clothing.) The sleeve length needs to be about 9". The short sleeved onesies have about a 2" sleeve; so I needed a 7" sleeve extension.
I used some old t-shirt scraps. I wanted to use the factory hems for the hem of the sleeve. I cut two 7" x 7" squares.
This is what they look like:
Before sewing up the sleeve, I checked the seam allowance and marked it with a pin. I didn't want it to be too narrow or wide for the original sleeve.
After stitching it up, I pressed the seam open. Don't skip this step. Then I turned it right side out and pressed it flat again.
To pin the sleeve into the onesie, roll up the new long sleeve a few times. It's narrow, so this part is difficult. Pin the new sleeve under the seam of the old sleeve. I wanted to sew right along the existing stitching so my new stitches wouldn't be very noticeable.
Sewing the sleeve to the onesie was a tricky step because the opening is extremely small. It didn't fit around the arm of my sewing machine. I was able to scoot the foot and needle through the neck side of the onesie. Be careful not to sew through any other part of the shirt.
Here's the seam. Steam the seam with an iron to flatten it out and you're all finished!