This is how to add a butt zipper to an existing onesie. The one bad thing about onesies is that if you need to go to the bathroom, you need to just about get naked. This especially sucks if you are out and about and in a bathroom or porto of questionable cleanliness.

Supplies

Onesie
Zipper of appropriate length
thread
pins
sewing machine

Step 1: Determine Zipper Location

Lay onesie flat or put onesie on and determine where in the back you want to put the zipper. On an animal onesie with a tail, this may be slightly off the best location due to the placement of the tail; determine if you want it above or below the tail. Take into consideration sitting on the zipper so you can avoid the zipper placement causing discomfort. My preferred location is right above the start of my bootie crack using a slight inverted u/v shape.

Step 2: Determine Zipper Length

Lay onesie out flat so the the center back is one side and the two side seams match up at the opposite side. Measure to determine zipper length (you will need to double the measurement since you are only measuring half of the total back). Buy matching length and color zipper. If you are wondering about the u/v shape described, well it lets you use a longer zipper and make a bigger opening, the easier to get yo bootie out.

Step 3: Cut Butt Opening

Once you have zipper , repeat step above and cut an slitopening in the onesie. Wait to do this until you have the zipper so the fabric doesn't stretch out around the cut before you sew in the zipper. It is best to try to cut a bit shorter opening than your zipper length.

Step 4: Pin in the Zipper

Pin the zipper in place to one side of the opening you cut, keeping the fabric edge at about the middle of the teeth of the edge of the teeth, depending on your concern of the fabric getting caught in the zipper or the zipper showing.

Step 5: Sew in First Side of Zipper

Sew in the pinned in side of the zipper.

Step 6: Pin in Other Side of Zipper

Pin up the other edge of the opening to the other side of the zipper. I like to start by lining up the center back seam and pining from there towards one side, then unzipping carefully and in sections as I continue the rest of the pining. Unzip carefully so the end doesn't unlatch if it's a separating zipper. You will want it unzipped before sewing.

Step 7: Sew in Other Side of Zipper

Sew in the other side of the zipper. Finish the zipper ends by bracketing them with straight stitching to prevent the zipper going too far and zig zag stitch up holes at the edge of the zipper if needed due to too large of an opening hole cut at the beginning. It is best to try to cut a bit shorter opening than your zipper. You are done.