I've seen alot of banjo students progress very well working with clearly defined arrangements only to hit a bit of a wall when it came time to arrange for themselves. The following is something I've used to break down my general banjo arranging formula for students. It uses Woody Gutherie's 'This Land is Your Land' as the example.
Step 1. Figure out the melody and the chord progression
You can do this a couple of ways. One way is to pick the melody out by ear from a recording, from another musician who knows the melody, or from memory. The other way is to find sheet music with the melody and chord progression notated.
Here is the straight melody for 'This Land is Your Land'.
And this is the chord progression.
Step 2. Begin to fill in the holes (spaces between melody notes)
You can fill the holes in any number of ways but chords tones (playing notes from the chords of the underlying chord progression)are a good first shot.
Filling in the holes with pinches (in this case plucking the outside strings with the thumb and middle finger) is perhaps the easiest way to start.
Here is 'This Land is Your Land' with melody and pinches.
Step 3. Experiment with rolls
Any tune can be arranged thousands of ways so which rolls you choose really is up to your judgement and taste. The forward roll (T,I,M) carries alot of power and lends itself to the kind of syncopation characteristic of Scruggs style. You generally don't want to overuse any one roll (in my opinion) but you can pick one roll to be the dominant, driving roll of the arrangement.
Here is 'This Land is Your Land' using forward roll and not much else.
Step 4. Final touches--Adding licks etc.
Once you are comfortable with the melody on the banjo and have experimented with filling the holes and using rolls you can begin adding the final touches. These may include:
1. Solidifying which rolls you are using and where---striving to find an appealing balance and breaking up any spots where one roll has overstayed its welcome.
2. Thinking of different places to play melody notes. Instead of playing a B note on the open 2nd string you could slide into it on the 4th fret of the 3rd string. Sliding and hammering into notes is an important part of the Scruggs style and can really make a tune come alive.
3. Adding licks where appropriate. As far as where and when to put licks that is again a judgement call but I would strongly advise keeping the melody as clear as possible.
Here is a Scruggs style arrangement of 'This Land is Your Land' using a few standard licks.
Hope that is useful to someone out there. Arranging is a personal and creative venture so any formula like the one I've outlined should be seen more as guide to help you along.