This is the basic forward roll:TMTIMTIM (Thumb Middle Thumb Index Middle Thumb Index Middle)
There are other varieties but this is a good one to start with.
Every measure starts with a Thumb which provides a good strong sound
It has a syncopation to it because of the three (TIM) over a 4-beat measure.
The hardest part is doing 2-3-3; then another 2-3-3
Here are three basic practice tabs. They use the same TMTIMTIM but hit different strings.
Here are the same rolls on the same strings but using a chord (either C or D7)
Here is a basic 3-chord song to practice the forward roll.
This is extremely basic for the first 6 measures. The trick is
to get the fingers switching chords without stopping the rhythm (just like when strumming or frailing)
In the 7th measure, things get a little bit different but not much. The last Index note is done on the 3rd string instead of the 2nd.
That can throw folks for a loop, too.
In the final measure, the roll stops early to let the final G note sound a bit longer.