The 7th-fret method is similar to the 5th-fret method for tuning a bass guitar, but it works in reverse (from high to low). You need to tune your G string (not your bathing suit, but the highest, or skinniest, string on your bass) to a reference pitch from a tuned instrument (if you're playing with others).
When you have the G string in tune, press it down at the 7th fret. The note you get when you strike the G string, with the 7th fret pressed, is D, but it's an octave higher than the next lower (thicker) string.
Here's a step-by-step description of how to tune your bass with the 7th-fret method:
Using a finger on your left hand, press the tuned G string down at the 7th fret.
Make sure you don't touch the adjacent (lower) D string; both strings should vibrate freely. (This brings a whole new meaning to the expression of fretting about your G string, doesn't it?)
Strike the fretted G and open D strings with your right hand and let them ring together.
The pitch is an octave apart, but it's the same note. Listen to whether the D string is sharp or flat, and then turn the tuning head for the D string accordingly until the strings are in tune with each other.
Press the tuned D string down at the 7th fret without touching the next lower string (the A string).
Strike the D and open A strings and let them ring together.
The pitch is an octave apart, but it's the same note. Again, listen to whether the A string is sharp or flat, and then turn the tuning head for the A string accordingly until the A string is in tune.
Press the tuned A string down at the 7th fret, making sure you don't touch the E string.
Strike the A and open E strings together and let them ring.
As with the other strings, the pitch is an octave apart, but it's the same note. Listen to whether the E string is sharp or flat, and then turn the tuning head for the E string accordingly. When the E string is in tune, your entire bass is in tune.