The trouble with installing pickups in a strat is the way the pickups are mounted to the pickguard can create difficulties. An easy option can be to buy a new pickguard (with a wider hole for the humbucker - no messy sawing of plastic) with the pickups already mounted on it. Then, what you can do is simply to remove the old pickguard and pickups and swap them over, wire for wire. This makes it a bit easier not to get confused, though it does make modifications to the basic setup harder. My squier standard series already has the routing required to put humbuckers in both the neck and middle positions, so you may find that you don't need to cut any wood out of your guitar.
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