You'll encounter three different ways to write down bass parts:
1. Bass tab
Four horizontal lines, one per string. G string on top, E string on bottom. Numbers on a line are frets to play on that string.
G|------------------|
D|--5-7-9----5-----|
A|--------5-----3--|
E|----------------- |
That reads: D string fret 5, fret 7, fret 9, then A string fret 5, D string fret 5, A string fret 3.
The advantage: shows exactly where to put your fingers. The disadvantage: tells you nothing about why those notes work. You learn the pattern but not the theory.
2. Chord charts
A chord chart just shows the chord progression with the rhythm marked above:
|| C | G | Am | F ||
The bass plays whatever feels right under each chord, usually starting with the root. As you get better, you walk between chords or add eighth-note grooves.
Most professional gigs use chord charts. The expectation is you know how to translate chord names to bass lines on the fly.
3. Lead sheets
A lead sheet has the chord names plus the melody (and sometimes lyrics). Common in jazz and worship music. Bass plays the chords; the singer/melodist plays the melody.
Which to use
- Learning a specific song: tab is fastest.
- Joining a jam or worship band: chord charts. Be ready.
- Studying a part: lead sheet or full score, since you see harmony + melody at once.
Practice translating
Take any song you know. Find the chord progression online (Ultimate Guitar is a free source). Write the root notes. Play them on bass. Now add walking between chords (1-3-5-octave from lesson 7). That's how a real bass player learns a new song.
Next: putting it all together.