It really depends on how much leather you've got, and how good you are at working it. The simplest breastplate pattern would look something like this :
Where A is an inch or two short of the width from armpit to armpit at collarbone height, B is the distance from the point of your collarbone to your waist, and C is half your waist measurement at the widest point of your torso.
From there you could either add a backplate (which would be the same sort of pattern, with a wider A as you would not have to compensate for your arms needing to reach forward) or just use belting to buckle the sides of C to each other and add either a single loop behind the neck from the A corners or a Y-strap that links them both to the C corners on your back.
The best way I've found to make a dirt simple pattern for this sort of thing is to toss on a comfortable disposable t-shirt, and cover it in a couple layers of duct tape, keeping the fit close to your body. Once the whole shirt is covered, you can remove it by cutting up the side seams, and there you have a quick and dirty blank to work from. After that, you just have to spend a bit of time trimming the edges inwards until everything feels comfortable when you toss it on, then trace that final pattern onto the leather and cut it out.
Here are a couple shots of the brigandine I used to wear, to give you an idea of how a breast and backplate can go together. The two narrow parts are shoulder armor, but beneath it is 2" wide leather strapping that has the breast and back plates suspended from it.
This second one shows how the armor sits on my torso, so you can see what kind of allowances we made for ease of movement.