The most difficult feature of bass note picking is the ability of the right hand to pick out the correct string. It is extremely important for the right hand to move mainly from the wrist, with only a minimal movement from the elbow. The following photos illustrate this wrist movement.
You may find it beneficial to practice in front of a mirror so that you can watch your hand movement. The speed and accuracy of your playing will depend entirely on your ability to develop this wrist movement technique.
This song is an extension of the bass note picking style of playing which you have been studying in exercises 18 and 19. Where eighth note chords occur (e.g., bar 2), the first chord is played with a down strum ( a ) and the second is played with an up strum( s ).
The chords should be strummed lightly so that they do not "drown out" the sound of the melody. Be very careful with the timing and be sure to hold each melody note for its full value.