What sort of plants are they? For stem plants, just stick them in the sand (you might find tweezers help with this). Rooted plants such as Swords and Crypts, dig a little hole in the sand, and bury the plant including it's rhizome. Having done that, pull the plants up lightly until it sits as it should (rhizome slightly exposed). If your having trouble with stem plants, you can always chop some of the bottom off, then chop the lower leaves so there is still a bit of "stalk" on them - then these act as anchorage when planted in the sand.
Hope this helps,
Rob.