Depending on the specific manufacturer they may have some online printable routing guides or patterns from which to make your own routing templates.
Stewart MacDonald also have range of premade templates for the purpose.
You're going to need to accurately mark a centre line on both sides to line things up. If you use the reference fret position calculator on the Stewart MacDonald site it gives you a reference distance from the nut for the mounting posts.

It's going to be a fair amount of very careful measuring, and you are going to need a good router, good quality carbide bits with bearings to follow the router templates and some accurate drilling and hogging out to make the routing easier.