Use this diagram to have the blueprint for your series/parallel wiring. Not sure what the color codes are on the stock pickups, but I guess that's it pretty standard Fender, with white-hot, black-ground and some sort of wire that series connects the two coils. You are just unhooking the P pickup's wire from the 3 way switch, manipulating the wiring as shown below and then sending the new 'hot' to the 3-way switch.
Get a push/pull pot to do the work and send that black wire that is coming off the bottom right side tab on the push/pull switch to the appropriate tab on the 3-way switch.
View attachment 3056127