Let's start out with a simple tune that we all know (at least in New Orleans that is)... The Saints Go Marching In.
Here we will play the melody strictly on the 3rd string (the middle G string). The first thing we need to do is to find the simple melody (no rolls - just the melody) on the 3rd string. We are finding the melody on only one string because by doing so, it is more visual and linear, like a piano keyboard.
After we have found the simple melody by ear on the 3rd string, we then will add a roll to it to fill out the spaces and play some of the harmony (the chords) to the tune to make it sound more "full". We will use the alternating roll because it is the easiest.
The first pickup notes where the melody goes "Oh when the" the melody is moving a little quicker than a full roll will fit, so we are going to use what I call "fill in" notes. The fill in notes are just going to be a D note played on the open 1st string. When we hit "Saints" in the melody, the melody holds on that D note for a whole measure (measure 2) and there is enough space to go through an alternating roll twice.
On measure 3, we need to use "fill in" notes again as we did in measure 1.
Go through the song and find where you are playing "fill in" notes and where you are playing an alternating roll.
Please ask questions in the comments field below and I will answer them. Good luck and have fun!