Find a romper or overalls pattern, and just do a big sack rather than the legs, or do this:
Make a large sack (like the magic potholder beginning)
Make a ch that is the width you want. SC or dc in each ch, then sc/dc around to the beginning (creating an enclosed bottom). Continue working in the round (or you can join or ch 1or2 to bring up to next row). Continue around to what would be the Under Arm.
*Working in Rows, rather than rounds, go back and forth until you get to the neck. Make straps: sc/dc on the first few sts leave remaining un worked, go up a few rows. Repeat for other strap.*
*Repeat for Back* Finish off. Sew on buttons and voila! You have a pattern..
Clear as mud?
LMK if you need help.