On the second string G, play A (Index finger) repeatedly in a loop with equal durations. Then after a couple of repeats, try to touch the string as quickly as you can with your second finger, which would be like a hammer-on toward Bb. While keeping that first finger firm, do not press the string too hard with your second finger, as we are not supposed to actually hear that Bb. The touch is just meant as a stop (mute) for A. Be careful not to make your main finger loose, or that A will faint away. Only after feeling comfortable with this, you can try the same on the other strings.
The next step would be to perform it on an open string, touching the string in between repeated notes on the open string with your first finger this time.