I measured where one armour plate would go over another, and holes were drilled in the brown 'channel's along the edges of each piece. Then I used strong brown thread to stitch everything together....
*Notice the sets of two drill holes on both end plates of armour. Those are for sewing leather straps, that will go under Matias' arms and hold the armour securely in place.
Finishing
I moved the buckles to the longer straps, and stitched the snap directly underneath. Then I stitched the other part of the snap onto the shorter strap where the buckle originally was.