When playing free stroke it is common to use i m and a in succession on the first three strings. Brace your thumb lightly on a bass string when playing this exercise.
Now try adding some notes with the left hand.
Here are some variations. Play each one many times slowly and evenly.
Once you add some notes with the left hand, this kind of playing comes alive.
Sometimes the thumb (p) is used on the 3rd string while i and m play the 1st and 2nd strings.
This example uses all four right hand fingers – p i m and a.