This is super easy and quick.
Pair of elastic band shorts that fit (either for you or your little one)
Fabric depends on what size you are doing anywhere from 1/2 yard to a yard. You can use a woven but I prefer a knit for this one.. that and I had 10 yards of a denim colored knit from the $1.00 store in LA !
Elastic enough for waist and top
Sewing stuff, machine, thread, scissors, etc.
Paper & Pencils
Lay shorts on paper and trace side seams and center front (crotch area). Front and back. Make sure when you make your short pattern you go straight up from the hem to the waist and don't taper in....the elastic will do that later. Add 1/2 in all the way around. If you are using a woven add enough for the hem. I'm using a knit so, I'll just be adding trim for my hem.
Measure (on the subject you are making this for) from where the shorts you traced (around their waist) up to where you want the romper to go. For me it was about 8in then I added 1/2. I would say next time I'll add a full inch. Then lay out shorts pattern on fabric (folded so you cut 2 of each pattern) and measure up the 9 in. cutting straight across the top.
Sew down center back and center front (that's the curved section).
Then sew down side seams, the long side, then the inner thigh side seams (see photo). If you need to trim up the hem line.
Adding elastic waist. For this I measured my daughter elastic shorts un-stretched. I had some 1/4in elastic on hand so I used that. Turn your romper inside out, then measure up from the hem the amount to where you want the elastic waist band to sit. For me I measure that from the shorts pattern I was tracing. Then pin the elastic evenly all the way around. use a zig zag stitch and stretch the elastic a little bit as you sew it.
Adding elastic top. Measure just under arm pit around the chest then cut elastic a few inches short of that measurement. It depends on the elastic you buy. I like 3/4 or wider for holding things up. I'll also be adding straps so I doesn't have to be super tight. For this I used 3/4 in elastic so I turned down the top 1 1/2 sewed along the bottom edge and left a 1 in gap to thread the elastic in.
Then you can add straps, sew on a ruffle, maybe some rick rack, I don't know? Get creative! My daughter found a red fake flower outside the other day so I pinned that on and she loves it.