This is going to be easier than doing things from scratch. Here is the complete pocket from the old 7th Regiment waistcoat. This style is typical for watch pockets in breeches. A tutorial on how to make these is here.
The new fronts have a chalk line the width of the pocket bag, but not as wide as the broadcloth welt seen on the left of this photo.
Here is the welt pinned upside down to the weskit front. After the welt is back-stitched to the slit, the pocket bag is pushed through. The pocket bag back is then sewn to the slit sides and top, the welt is folded up, pressed, and the welt sides are back-stitched. Easy.