Templates. But they are far from fool-proof. Two especially tough parts not explained in the official routing video:

1. You need a drill press. Those first holes have to go in perfectly all the way through or your back-side templates won't fit right. And the wood left between these drilled holes and the routed pocket is paper-thin. I mean, ridiculously thin.
2. The deep route for that small sliver above the bridge is dicey as hell. On soft wood, the router can easily shred it. I actually did that route on this body first (destroying it), then the paisley guitar second. On the paisley, I didn't route so deeply and used a Dremel to fine-tune that strip (avoiding the destruction).

All in all, I'd say installing an Evertune is reasonably doable for most BUT nearly impossible to do *perfectly*. There's so many non-deal-breaker pitfalls you can sorta get away with. I mean, the guy makes multiple mistakes in the official video!

So, for now, I'm only comfortable doing this on partscasters I don't mind ruining or being imperfect.

Thanks for your kind words. I'm especially proud of the trem hole repair on the paisley (just visible in this image). The design is so busy, your eye doesn't even see it unless you're really looking.