To go from standard to drop d : tune the 6th string down so when you play the 6th string 7th fret, it sounds the same as the open 5th string. For drop C, start at drop d, then tune strings 1-5 down to standard according to the 6th string (now a d) Then tune the d down so the 7thfret 6th string sounds like the open 5th string (now a g).

Edit: But it would be a good idea to purchase a tuner.