Make sure your bass guitar is already ready. The default setting for a four-string bass guitar is just like the default setting for a six-string guitar. The notes are, from the fourth (lower or thickest) string; E, A, D, G. 2.
Set your bass by using the corresponding keys on the keyboard or by using a handheld tuner, which you can buy at any good music or guitar store.
Pick the second string, D, to get the sound in your mind. You will set the fourth (low-E) string to match this.
4. Play the 12th harmonic note on the low-E string by holding your finger lightly on the fourth string directly over the bar on the 12th Fret (not to the left or right of it, as you normally do when playing singles notes), picks the string with your pick and immediately lifts the finger from the string. When you hear a ‘ping’ or a ‘bell’ tone, you know you have the harmonic right. It takes some practice, but you can figure out the time if you are patient.
Set the low-E string while the harmonic note rings so that it matches the tone of the other (open D) string. When both notes are played together, they are heard to collapse if the notes are suspended from each other. The less they collide, the stranger your sound is and you know you have the string set correctly and you can play your bass guitar in drop-D tuning.
If you want to get into the sound of today’s rock and roll, you may want to experiment with the ‘drop-D’ setting, which in some accounts is at least very popular tuning for bass guitar. It is very easy to do and you only need to take your head around the difference in sound when you do your finger upside down on the table.