In my case I want to install humbuckers in an American Stratocaster equipped with single coils. The routs in the body are already there from the factory, so the only thing in the way is my pickguard.
The best thing to use as a template is sanded 1/4 inch plywood from Home Depot. this is the best material I have found that offers ease of use and low cost. Take your old pickguard and trace a line around it were you will cut the template, also mark the location of the screw holes, hardware, and pickups, these will help.
Use a coping saw to cut on the line around the pickguard. It's better to leave a little edge around the template and then sand in afterward. I did a lot of sanding to get my template to be perfect, the better the template looks, the better the pickguard is that will come from it.
Next, measure the location of the pickup routs. I had to match the location of the pickup routs to my guitar body perfectly so that everything would fit right, so I used the pickguard screw holes as a reference since they determine where the pickguard would sit.
Once I had the location traced on the template, I used premade templates from Stew-Mac to get perfect pickup holes in my teplate. These require their ball-bearing router bits, the same as we will use on our wooden template. Use double sided tape to hold the template in place and drill a hole the size of the router bit to get things started.
Once everything is routed and sanded, we can attach our template to a piece of pickguard blank material. Use a coping saw to cut within a quarter of an inch of the template, the finer the blade the easier it will be to cut the vinyl. Drill holes in the pickguard where you will be routing for the pickups:
This is a picture of a premade acrylic Tele template being used to make a pickguard on a table router. notice how the pickguard is trimmed to about a quarter of an inch from the edge before the table router is used. This is to prevent the blade from binding and ruining your pickguard.
Then use a beveled router bit to get a slanted edge around the outside of the pickguard. Use a straight router bit to get all the straight edges.Here you see the new pickguard near completion with a humbucker installed.