I'm assuming that your guitar is currently in Standard tuning ( E A D G B e ). Here, 6th refers to the thickest and 1st to the thinnest.
1) Strike you D string - (4th) and then loosen your E string (6th) till they sound the same.
2) Leave the A string (5th) as it is.
3) Strike the note on the fourth fret of your D string (4th) and loosen your G string (3rd) till they sound the same.
CAUTION: G string need not be taken literally!
4) Strike the note on the third fret of your retuned G string (3rd) and loosen the B string (2nd) until they sound the same.
5) Finally, strike
the D string (4th) and loosen the E string (1st) until they sound the same.
Congratulations. Your guitar is now in open D tuning!
If you play all the strings open, you will hear the D major chord.
Technically, that would mean your guitar is currently tuned D A D F# A D.
For more info
about how Richie makes use of this tuning to play his chords, take a look
at his official
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