The finger roll is notated as a group of notes with a wave-like line next to it. Sometimes this wave-like line is notated with an arrow indicating the string order to do the finger roll in. If you don't see an arrow, do the technique in the usual order, from thumb to ring.
The finger roll plays multiple strings in quick succession using the fingers on your right hand. Similar to a downwards strum, multiple strings are picked in quick succession.
Starting from the
lowest string of the group of strings, pick each string in the order of thumb, middle, index, and ring finger, with equal timing between each string. If you encounter a finger roll with less than 4 notes, use the fingers that are most convenient to do the technique with.
You can also do the finger roll in a reverse order, starting from the
highest string of the group of strings. However, this is rarely used and slightly more difficult.
Your end goal should be to do a finger roll without having to do the following. Think of the following as training wheels that may help you get used to the motion before moving on to the proper form.
Once you're comfortable playing the finger roll using these methods, practice the finger roll without them.