Once you've established what size you'll need to best fit your frame, take your armor templates and trace them onto some plastic ( if you're going that route ).
Once it's all traced onto the sheet plastic, cut it out. Then you'll have to shape the plastic to acheive the desired curvature... again, to best suit your body. I used a big pot of boiling water to heat the plastic. Once removed from the boiling water, gently make your curves and hold them in the desired shape while you plunge them into cold/ice water. ( this will set the plastic into its new shape. )
There are more than a few other methods that you can use, it all depends on what material your working with.