I've made a few from scratch, without a pattern or template.
I suppose the main issues are to do with the fabric in how and which way it stretches; this also affects the banding/edging.
Given the stretching issue, you'll also need to choose your stitch to suit.
Other than that, it's fairly straightforward and only a little more complex than a T-shirt.
Well a cheap and easy one is to get a regular t-shirt. Be4 i forget measure from ur shoulders to ur crotch with a diaper on. For me it was 2 sizes bigger then get a set of snaps (walmart carries a set with the baby style snaps and reg snaps with the tools to install for arroumd $5, if my memory serves me right) lay the shirt on ground and put snaps in the center, i used 3. On the down side they are plain and not babish