Well, *the* very best DIY method would be for you to copy a guitar that has a full-size humbucker route and pickguard. You'll want to make a template, perhaps out of 1/4" MDF (for a one-time use only sort of deal) with a router. Here's a pic of the 2 bits you'll wanna use:
Take the MDF, and set your Mustang pickguard on it. Trace it out, including all the screwholes and the slots for the pickups. Using a 5/8" drill bit, route out the bridge pickup area of the MDF, and use a 1/8" bit to drill out two of the screwholes *furthest* from the bridge pup. Use a 5/64" drill bit for all the rest of the screwholes. Set the Mustang pg aside, and grab the pickguard that has the humbucker. Line it up on your MDF template and secure it with screws. (The two 1/8" screw holes are for securing the MDF to the body, the rest are for securing the pg to it) Using the little 2-flute bit you see in the pic, route it out, using the pickguard as a guide. Make sure the guide bearing rides the pickguard, or else you'll have a mess. (You can bypass this step if you feel confident enough to free-hand it, I usually do). Now that the MDF is routed, install the Mustang pg, swap bits and repeat. Your Mustang pg is now routed for a humbucker. Now for the body. Repeat the pickguard process for the body. Done.
