I’m putting a prefab glass block window in a remodeled shower. There was no window in the old shower area. I’ll be cutting out the window space, then will build a 2 x 4 frame. I have Clapboard lap siding on the outside.  I was going to cut an additional 4″ of lap siding around the outside opening to allow for window trim.

My question is how to best Flash / waterproof the outside?