Its not too difficult, more a case of taking your time and double checking measurements.
First off you need the centre point of the bridge, get this from the middle of your pickup route, then you need to measure 25.5" from the INSIDE of the nut on the neck (temporarily put a neck in place to find this measurement).
From this you have a horizontal centre line for the thimbles and also a centre point for the middle of the bridge (between the G and D saddles).
The thimbles are usually pushed into place tightly (on CIJ models at least) so you need to drill two holes possibly 1mm under size with a Bradpoint drill bit (these dont wander off-centre).
You may also want to consider grounding at least one thimble, drill a small hole between the control cavity and thimble hole so you can connect a ground wire.
If that all seems confusing you could try asking Mad-Mike or Joey, they've built JMs and may be able to email you a template jpeg image. 😉
