The free stroke is different to the rest stroke in that after striking a string the right hand finger does not rest on the next string, but instead moves freely over that string, as shown in the photos. You have already learned free stroke with the thumb (p). Free stroke is commonly used with all right hand fingers, particularly when several notes are played in succession on different strings.
This exercise will help you gain control of free stroke. Brace your right hand thumb on a bass string to help keep your right hand steady. Once you can play this example with i and m, practice it with m and a as well.
The next step is to add some notes with the left hand. Keep each left hand finger pressed down on the string until it is needed for a new note in the next bar.