I bought this for my guitar to give it a good clean up.
It should be used on the fretboard only, applying it to the guitar body or anywhere else that is lacquered will just leave it greasy.
I have a rosewood fretboard and using this and a microfibre cloth, left it with a beautiful clean lustre.
Some lemon oils come with warning saying that they shouldn't be used on maple fretboards, this one in particular does not have a warning on it, but I would still do some research before thinking of using it a maple fingerboard.
The easiest and most convenient way to apply the oil is to do it when you are restringing the guitar. when I did it, I removed two strings at a time and put a few drops onto a microfibre cloth, I then worked my way down from the top fret, put some more oil on the cloth and worked my way up from the bottom. After that, I used the cloth on each fret individually (no need to apply more oil as there will be some remaining on the fretboard) and cleaned with side to side motions.
You know it works because you can see the dirt on the cloth afterwards, and you'd be surprised by just how much there is!