screw length is crucial! dont go driving screws out the back of the body.
to start you will need something to keep the pickups at height. some people believe you want a good sonic transfer from the body and will use a wooden shim and fasten the pickup rigidly. if you do that you will need to precisely measure to get proper pickup height or make multiple shims. you can and many people do use a foam block under the pickup for adjustability.
now to hold the pickups down you will need either smaller screws than used in the pickguard or drill the pickup mounting holes. there may be very little wood under the mounting tabs and therefore little room for adjustability. you will need to measure the height from the top of the p/up to the top of the tab and measure how high the pickup protrudes from the guitar in the lowest position you might use. subtract that and your value will be subtracted from the thickness of the guitar. this value will be the longest the screw can be before it breaks out the back of the guitar. now if you have a trem the spring cavity will be under the bridge pickup, measure the spring cavity depth and subtract that from the previous value and this is this is how long the screws can be, make them a bit shorter for good measure unless you dont mind them showing in the spring cavity.
you should have pilot holes to avoid cracking. the pilot holes should be a size smaller that the major diameter of the threads or a size larger than the minor diameter, you don't want to match it too the minor diameter, the wood will displace into the threads. dont force it. you may also want to run machine screws and t-nuts or other inserts. if the pilot hole dont hold the thread in a soft wood you can fill them with glue.
an alternative but not a popular one is to drill through the guitar, running the screws in from the backside. you can either run countersunk screws and chamfer the top of the holes or counterbore the screws and put a wood plug over them to make them invisible.
