So I just noticed that there is a tutorial for something very close to this. Mine is similar, but there are differences...
For example, I didn't use the neck/sleeves unaltered (because I was using a bigger shirt), and I did the crotch part pretty different.
Here is my first attempt- I used an old cruddy tshirt first so I wouldn't muck up a cute one:)
Here is how I cut out the sleeves/neck. Just like in the other tutorial, I used a jumper that fit well, but I didn't use it to cut out the bottom, only the neck and arms. For this one I made the sleeves a little shorter to avoid the shoulder seams (the shirt is a women's med, I believe, and form fitting). For the bottom I just cut clear down to the seam, then sewed the sides first (makes it kinda look like a night gown
And this is how I figure out how to do the crotch:
I put the "nightgown" on my "model" (my normal model was sleeping while I was trying to take pics to post, so bunny is his stand in- this is a role bunny often fills, as he is pretty darn close to the size of a baby) and pulled the back up through the crotch, like so...
Then mark where to cut the back and the front with a pin. Does that make sense?
Then I cut the FRONT to look like this, leaving enough to serge the edge then turn and topstitch (cut the FRONT only- the reason you can't see the back is because I pre cut the front using my previously made jumper as a template)Cut the back straight across but long enough to reach through the crotch as before.
Then I just serged the edge, then turned and topstitched all around. Looks like this finished:
And then I am going to put snaps in the crotch, but I haven't bought snap pliers yet!