You want your left hand to cover one fret per finger without causing any undue stress. By using one finger per fret on your bass guitar, you set up your hand to execute almost any musical figure without shifting, or moving your hand position to reach a note. (A figure is an independent and self-contained musical phrase, sort of like a sentence when you're speaking.)
Check out this demonstration of left-hand positioning in Chapter 2, Video Clip 1.
When you do have to shift your hand, a move of one fret in either direction usually suffices.
Follow these steps to accomplish the proper left hand position:
Hold out your left hand with your outstretched arm in front of you.
Keep your wrist and hand limp.
Without changing the angle on your wrist, turn your hand so your palm faces up and your fingers are slightly curved.
Position your thumb so it faces your index finger (or the area between your index and middle fingers).
Bring in your elbow to the side of your rib cage (without moving your hand) until the neck of your bass is in the palm of your hand.
Remember not to close your hand!
Place the tip of your thumb on the middle of the back of the bass neck.
Make sure that your fingertips are pointing upward.
Gently spread your fingers onto the strings, with each finger close to an adjacent fret.
Curl your fingers until your fingertips are on one of the strings.
Be sure to keep the tips of your fingers close to the frets.
Now you're just about ready to press the string to the fret to play a note. Even though you can now fret the desired note, something still has to set the string in motion to produce the actual sound. This is where your right hand comes in.