Now that your backing plate is installed and cured the first thing to do is to drill your hole for the thru-hull. This is made easier because you have already centered it over the old hole and already have a pilot hole in the center of the backing plate left over from the large hole saw you made the backing plate with. Now you simply choose a hole saw, to match your thru-hull's diameter, and using the existing pilot hole, drill your new mushroom-head / thru-hull hole.
Once that is done you'll need to thread the thru-hull up into the flange and measure it for length. In most cases the thru-hull will be too long and need to be cut to size. Cutting the thru-hull stem is easy.
1- With the thru-hull bottomed out into the flange measure from the outside of the hull to the mating side of the mushroom-head and add 1/4 of an inch.
2- Insert the thru-hull into a vice carefully clamping the threads that need to be cut off NOT the threads your keeping.
3- Using a hack saw cut the thru-hull to its required length.
4-Clean the threads you just cut with your drill and brass brush.
Now that you have the proper length thru-hull, re-insert it into the flange and tighten it down by hand. Climb back into the boat and adjust your seacock to align it with it's hose for proper orientation. Once you have everything aligned, and situated where you want it, drill your flange bolts holes, one at a time, inserting a bronze bolt after drilling each hole. Inserting a bolt, after drilling each hole, guarantees proper hole alignment by the time you get to the last one! Doing it by eye, without bolts, leaves room for the seacock to twist and a potential fit failure. There is NO room for error with fiberglass like there is with wooden backing plates and all bolts must be 90 degrees to the hull and perfectly centered and aligned for a proper fit.
The next step is to countersink the machine screw heads into the hulls exterior. Again having the proper tool, in this case a countersink, is imperative. Not only must you use a countersink but it needs to be the right size for the silicon bronze slot head machine screws! Using a regular drill bit for countersinking is a HUGE NO, NO in this situation! In the picture the holes have been properly countersunk and the through-hull hole, in the hull, is over an inch thick of solid fiberglass.