Begin with your guitar tuned to standard tuning (EADGBE).
Tune your guitar to dropped D. The simplest way to do this is to pluck the D string and then lower the E string until it is the same note. The low D will be an octave lower than the D string.
Re-tune the rest of the strings like you would for standard tuning using the low D as the reference. To do this, start on the fifth fret of the lowest string (originally E, now D), pluck it and lower the next string (originally A, lowering to G) to that pitch. Repeat on the next 2 strings so that you have D-G-C-F.
Hold down the fourth fret on the fourth string (originally G, now F) to reference the pitch for the fifth string.
Change the pitch of the sixth string by holding down the fifth fret on the fifth string and matching that pitch by lowering the sixth string. Your guitar should be tuned down a whole step to DGCFAD.
Drop the tuning of the lowest string down one more step. Do this by tuning the guitar to dropped D like you did in Step 3, but it will be a low C now. Verify it by holding down the seventh fret and comparing it to the second string. It should be the same pitch.