Here's how I do mine. This is the strap arrangement for the inner apron, so It's all hidden. I do the over-apron the same way but use a bit of the tartan fabric (or whatever your material is) for the strap.
A good camping gear store will have loose nylon strap keepers. Some are flat and some have a gentle curve to fit better against your body.



I've improved the design a bit by attaching the strap 2" from the edge of the apron rather than right at the edge. That way when you cinch it down the nylon fixture lies on the inner apron rather than against your shirt for a bit of extra "padding".