Here is the bridge doctor before installation
Here is the action before - about 5mm on the bass side and about 3mm on the treble.
The guitar on the workbench
Hole drilled - I got a 1/4" brad bit for the outer hole and used a cocktail stick marked at 1/8" as my depth gauge. You get a little plastic dot you can inlay over the screw once finished - I may do that later when I get happy with it. Luckily I have a local little hardware shop that stock imperial drills and are cheaper than the big stores... no doubt as the stock has been there 20 years!
Set the doctor to the best width - the circular plastic bolt holder has three settings - I used the widest you can see to allow string clearance on the 12 string - on a 6 string the middle would probably be best setting.
The doctor installed - the rod needs to go in and the allen key bolt on that and you tension it there. You need a 3/16" allen wrench - I have all kinds courtesy of my Dad's old tool collection!
Sadly after all this work - to be honest even with a fingerboard clean and oil and fret polish it was only an hour or so - the action is only marginally improved. I tried slacking off the strings again and tensioned it more but with little result. Possibly my poor old Yamaha is beyond saving other than a full neck re-set which frankly will cost much more than the guitar is worth and I'd be better off buying a new electro one which would be better for live use anyhow. However, next time I restring it I'll un-tension the bridge doctor then I'll try to pull the rod down and re-tension it against that new position - which is a way I've heard of some doing it i.e. stepping the rod down the end block.
However the claim that it improves the volume and tone of the guitar definitely sounds true to my ears.
Given that the guitar isn't really saleable as is and this whole job cost under £30 and a couple of hours I still think it was worth a shot.