first, find the positions/sizes of the mounting tab thingies. Using the correct sized bit and placement, drill those holes.
Second, find as accurately as possible the dimensions of the pickup. Add the diameter of your router's base plate to the dimensions. Now, using the new big dimensions, clamp to the face of your guitar guides (straight pieces of wood will do) at those dimensions, centered around your chosen position.
Find a router bit that matches the corner radius of your pickup. You will use that for starters. Place the router bit in your router, and just use the frame as a guide. You will end up with an outline of your cavity, with the mounting tab holes already at full depth! Do this, advancing 1/4" depth each time, until you are at full depth.
Now switch to a larger bit, still keeping the frame in place, and hog out the rest of the wood in the middle 1/4" depth at a time. When you are finished, remove the frame. Voila, a perfectly routed cavity!
Or, you could use a pattern bit with a bearing, if you have access to one.
Just be sure to measure carefully, and make sure your frame is square at the corners.
Good luck!