This is very simple to do.
I measured 11" from the top of the shirt along the buttonplacket and cut off the bottom.
On my grandson, I needed 5" of the bottom of a onesie in his size. 24mo.
i sewed the onesie bottom to the bottom of the shirt right sides together. make sure
you put the front of the onesie on the front side of the shirt :)
These will stay tucked in when you want the little man to look his best....