Notes on the 4th, 5th and 6th strings are most commonly played with the right thumb (p).
Notes played with the thumb are usually played free stroke. The thumb strikes the string but does not come to rest on the next string.
The basis of the free stroke is a circular movement. Play the string with a downward motion and as with the rest stroke, to the left side of the thumb nail, then bring the thumb back to its starting position. Rest your i, m and a on the three treble strings to support and stabilize your hand.
This example demonstrates the notes D E and F played with the thumb (p). Play it slowly and evenly and count out loud as you play.