Starting with a tank top under piece, you pin it until it's form fitting. Much like the top of the dress is in this photo.
All of my dolls are hand stitched so it would make sense the undershirt would be hand stitched, the one problem people will have is needing to leave one side open to make adjustments. It's a bit of fan dangling the fabric to make it fits but when your done you can stitch the open side while it's on the doll, stitch it closely.
Once the chest was done I move onto the legs. The prior owners had bent the long skinny legs in the middle so the hay was ruined and still felt apart. She lacked a lot nearest the body, even with the material being complete and no holes in it for the hay to spill out. I took the seams apart and added shavings to bulk her up some.
Then she was ready to construct. The pantaloons slid on and were stitched to the body.