Many stem plants already come bundled with lead sinkers. It is advisable not to use them, however, as they tend to damage the stems..inturn causing the stems to rot at the site of damage. I would avoid tying them together for the same reason. Just push them far enough into the gravel to the point if you give them a slight tug, you feel some resistance. If they aren't staying down, then they aren't deep enough. Just make sure you clean the lower part of the steam of all leaves, before burying it in the substrate. I normally strip about 3" off the stems, myself. Once the plants have developed roots, getting them to stay down is easy.