This is a fun tuning. It is known by several names including “Double Drop D” and “Double D Down”. It is identical to drop d guitar tuning, except that the highest E is also lowered to a D. Hence the Double D in the titles.
I am assuming you have your guitar tuned in standard guitar tuning. Pluck the D string (4th) and the low E string (6th) in unison and then lower or loosen the sting, until the pitches are the same.
Use the 4th string (D) to get the right pitch. Loosen the peg on your thinnest string until the pitch is the same as the D.
You should now have three D strings: the sixth, fourth and first.
| Strings | Standard | D A D G B D | Changes |
| 6 | E | D | Lower 1 Whole Tone |
| 5 | A | A | - |
| 4 | D | D | - |
| 3 | G | G | - |
| 2 | B | B | - |
| 1 | E | D | Lower 1 Whole Tone |