4th string D tuning note

When using the basic tuning method for standard tuning, you would play the 5th fret on the 4th string and tune the 3rd string open to that. In Drop D tuning, you are going to play the 7th fret on the 4th string, then tune the 3rd string open to that. From there, you will tune everything the same as standard tuning. You could also tune the rest of the strings using harmonics.

Basic relative tuning for Drop D

Page 1, getting into Drop D from standard tuning