i searched for a long time when i was going to install my FR in my explorer
the method i came up with is crude, but works.
what i did is get some yarn, and tie nots at the end, then strung it through the FR at the low and high E, and tied it off at the tuning machines, and placed them through the nut. i then used that as a guide to get the FR centered right. after that, i measured both ways from the 12th fret to get it centered that way too. after that, i just traced around it and routed it, about a 1/2". then i just marked where the posts went and drilled. its fairly easy to do if you know how to use the neccicary tools.
after that, i noticed that when i went to pull the bridge forward, it hit on something, so some more routing was required. all in all, not that hard, just frustrating