Practice, practice practice. As a beginner, it's easiest to make the stripes coming towards you. Use your wrist to flick, not your brush. If you're trying to make straight lines, dab only the tip of the brush with paint, anchor your pinky and drag the brush across as if you're flipping your hand over. Acrylic paint is best if you're having trouble as you can just wipe it off and start again til you get the hang of it
