Take off anything that's wrapped around them, doing your best to get it all off without damaging the roots too much (I know this can be difficult, just do your best). You can trim them by a third; that encourages the plant to make new roots and settle in faster.
Hold the plant between your forefinger and thumb, put it in the tank and use your other fingers to make a hole in the sand. Put the roots in and, still holding it, use your free fingers to fill in the hole.
Make sure you don't bury them too deep; you want the 'crown' of the plant, where the leaves come from, to be slightly above the sand. I find it's best to plant them too deep to start with and then pull them up slightly.
If they keep floating away, you can pile some pebbles around the bases for the first few weeks, until they've rooted in.