I take it that there have never been any tuners installed before and there's no holes drilled into the neck.
Mark out the g string position by placing black electrical tape from the nut slot to the top of the headstock, then taking into account where the string goes around the tuning peg, offer up the tuner to the headstock the wrong way, like as if you were goin to put it through the front of the headstock, and then mark the placement with a centrepunch when you're happy. Drill the peg hole, then the securing screw holes, install it and repeat.
Use the centrepunch to mark out all the holes, you dont want to slip while trying to drill those holes!
