G string to F#

Can't you just tune it to the 4th fret of the D string? My brain is a bit scrambled right now and my theory might be a bit off.

fada wrote:

How do I tune it and can I use a capo to tune it

I take it you're wanting to put your guitar in a D major slide tuning (DADF#AD). I would tune your guitar before putting the capo on, first. If you had an electronic, chromatic tuner you could just hit your G string and tune down the equivalent of a fret until you're in F#. Otherwise, you want your string to sound the same as the note you get when you pick the D string on the fourth fret. So you're tuning your G string down ...

When you put your capo on, depending on the make, you might want to fine tune your guitar. A cheaper capo is likely not to hold the strings down hard enough; even the best can be unreliable if you've got things wrong with your guitar or string setup. If your ears aren't the best, a chromatic tuner is perfect for this. Get one. They're very inexpensive.