The rest stroke (indicated by d), is so named because the finger having plucked the one string, comes to rest against the neighbouring string.
Grip i on the first string, making sure the right hand is correctly positioned.
Pluck the string by allowing the tip joint to collapse and come to rest on the second string on the flesh approx. ¼" (6mm) from the fingertip.
Relax the tip and allow i to return to a position close to (but not gripping) the first string, ready to strike again.
Repeat this exercise with m and a, remembering to keep the right hand as still as possible