For those of you that don't know a onesie is a t-shirt with snaps in the crotch. You can pick the up everywhere for babies but for an adult you need to order form a specialty dealer. They normally start at $25 plus shipping and go up from there.
Step 1: Materials
For this project you will need.
1. A T-Shirt a few sizes too big. I wear a size large and used a 2XL
2. A t-shirt that fits for a pattern
4. Sewing equipment. I sewed mine by hand but a machine would be faster.
5. Pins. Lots of them.
6. A marker or chalk for lay out.
7. A tape measure
8. Snaps. I used sew on snaps but Velcro could also be used. (9mm shown in picture have since been replaced with stronger 21mm)
Step 2: Layout
1. Turn the large t-shirt inside out and spread out flat.
2. Lay the smaller t-shirt on to of it with the tops aligned
3. With the marker or chalk draw a line straight down from the armpit seam of the large t-shirt to the bottom along the smaller shirt on both sides.
4. Take your round object and trace a radius from the line you just drew to the armpit of the t-shirt.
Step 3: Pinning
1. While keeping everything flat put many pins along the lines on both sides all the way to the bottom.
2. Carfuly slip the shirt on and decide where the split for the legs will be and mark it.
Step 4: Sewing the Sides
Sew from the armpit all the way down to the mark you made for the split. It is a good idea to re-enforce the bottom of the split with some extra stitching. I use a double row of hand stitching.
Step 5: Cutting
Leaving a centimetre or so of extra cut along the outside of you seems.
Step 6: Laying Out the Crotch
1. Fold the shirt in half.
2. Decide how wide you want the crotch and divide than number in half.
3. Make a mark half the desired width from the centre.
4. Lift up each layer and transfer the mark to it. There should be four marls all the same distance from the centre.
Step 7: Pinning the Crotch
Step 8: More Sewing
Sew along your line of pins to form a hem.
Step 9: Triming
Cut off the triangles of material to complete the hem.
Step 10: Putting on Snaps
1. Turn the t-shirt back right side out.
2. Turn the bottom edge of both sides up. Arrange them so they are close together. THis way you will be able to see where to place the snaps on both sides.
3. Sew the first snap on both from and back.
4. snap the together and leave them that way while you do the others so they will all line up.
Step 11: Finished
To improve the fit next time around I will be starting with a larger shirt so I can position the snaps more to the front rather than in the middle. I would also not make the front and back identical but rather the front slightly narrower.