As the music you learn becomes more complex, it is sometimes difficult to work out which left hand fingers to use. Where this is not obvious, a composer or arranger will often indicate the fingers to be used by placing small numbers near the notes.
Remember that the left hand fingers are numbered as such:
Although open strings are usually obvious, they can be indicated by a small 0 near the note. Notice that the note G is played by the 4th finger even though it is at the 3rd fret. This is common in Classical guitar playing.
When playing free stroke with the right hand, it is often desirable to have notes sustaining along with previous notes. To achieve this effect, continue to hold down the left hand fingers until they are needed for a new note.