The tuning for this song is similar to a variation of open G (DGDGBD) that has the bottom string tuned right down to a very low G. The actual key for this song is Ab though, one semitone above G, so the tuning you need is Ab Ab Eb Ab C Eb.

Joni's tuning notation makes things easier to get to the right tuning (an explanation of Joni's tuning notation is also available from this site). The idea is to tune one string by fretting a note on the string below - the umbers in the notation tell you what frets to use.

In this case, with the very low pitch for the bottom string, it's probably better to tune the 5th string to Ab, then play this string at the 7th fret to tune the Eb string, then play this at the 5th fret to tune the next string ... and so on. At the end, you can tune the bottom Ab to the other Ab strings.

Alternatively, to get to the correct tuning from standard tuning, try this:

* Tune the A string down to a G (compare with the G string)
* Tune the top E to a D (compare with the D string)
* Tune the low E all the way down to G - check with what used to be the A string, but is now a G.

This gives you the open G tuning with the low G at the bottom. If you put a capo on at the first fret, you're then in the tuning for "This Flight Tonight".

I've written out the chord shapes at the end - you might find it easier or quicker to learn the song from these if you know the song pretty well already. If you're not so sure about strumming etc, I've tried to give the outline in the TAB.