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 here.
This exercise will help you gain control of free stroke. Brace your right hand thumb on a bass string to help keep your hand steady.
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.