On an earlier post, I wrote about baby pants and knickers. I have used Dana's Perfect Diaper Cover pattern and tutorial to make romper suits, too - works perfectly. You can find her pattern and tutorial here. Footnote 2015 - she has a revised pattern now but it is also on her website.
All I did was extend the height of the waist to armpit level (by measuring the baby!) and adjusted the 'waist' elastic to chest size. Bright yellow double-fold bias tape went round the legs and top edge to form a casing for the elastic. Then I added four straps, two front and two back, to make tie shoulders.
I love this material, it's bright with weird monsters or gonk-like creatures on it. So I also added a little pocket at the front, with more bias at the top edge, and I used one of the monsters cut out of the material to make a gonk to keep in the pocket. It was padded with a tiny bit of wadding, and attached to a string sewn inside the bottom of the pocket - so it could come out but wouldn't get lost or swallowed. Though of course it did get a good chewing, and eventually the tumble dryer did for it. Never mind, it provided a nice outfit for sunny days.
I will certainly use the method again next summer. Maybe I won't bother with a pocket accessory!