Tune each string down one semitone from standard tuning
6th string: E down to Eb
5th string: A down to Ab
4th string: D down to Db
3rd string: G down to Gb
2nd string: B down to Bb
1st string: E down to Eb
Use a tuner set to chromatic mode
Tune each string until the tuner shows the flat note
Check the strings again after tuning all six
Re-tune any string that drifted
Verify the final tuning as Eb Ab Db Gb Bb Eb