grab your bass, flip 12th fret harmonics on your D string and E string, grab your E string tuning peg and detune until the two ringing harmonics are in tune with each other.. check the tuning by comparing your open detuned string with your A string fretted at the 5th fret
...and that's it.. you now have a note two semitones deeper (two frets worth) on your bass, for free
the main thing to be aware of is that your normal patterns across strings won't apply because the interval between the low D string and the A string is a 5th... wheras normally on a bass it's a 4th
you shouldn't normally need to adjust the setup of your bass to take into account the decreased tension in your strings...