I have installed a Bigsby
on a Les Paul and a B.B.King "Lucille".
It is important to get it perfectly aligned
before drilling holes.
loosely attach the Bigsby to the end of the guitar (on Gibson guitars, where the strap peg is located).
Take strong string from the 1st tuner peg, down the neck to the
tailpiece to where the 6th string attaches,
across the tailpiece to where the 1st string attaches and
back up the neck to the last tunner peg.
Tighten both tuner pegs, then insure both strings going up/down the neck is perfectly aligned, then mark where to drill the holes.