I would use tweezers like coach_z described. Another way would be to unbunch them, strip/remove dead leaves & stems, then rebunch 3-4 of the stems with the weight. Don't stick it in the substrate, but let it just sit on top of the gravel. Within a week, the plant will begin to grow some roots to attach itself. Once there's enough roots for anchorage, you can remove the weights and replant them individually. Rotalas and Ludwigia will send out roots when they're above soil. Of course this only works if you don't have large fish that will move unattached plants around.