marimo ball / moss ball

Drop them in the water, preferably in a current, make sure they get plenty of light and every time you do a water change make sure that they roll around.

There are three growth forms. One is epilithic (growing on rocks) and is usually found on the shaded side of the rocks. Another growth form lives as free-floating filaments, as small tufts of unattached filaments that frequently form a carpet on the muddy lake bottom. The third growth form is the lake ball proper, where the algae grow into sizable balls of densely packed algal filaments that radiate from the center. The balls do not have a kernel of any sort.

I'd have to agree with everyone, but I would add that you can also grow these attached to wood.

Like everyone says, it depends on how you want them to grow.

They can be trained like moss to grow on rocks and wood. They can be left sitting still and turned from time to time. Or they can be in high current tanks making an entertaining display of rolling around the place.