Riccia fluitans, also known as Crystalwort, is a beautiful plant. It is by nature a floating plant but many people, including myself, like to tie it to driftwood, rocks, or aquarium decorations with thread-- or better yet, fishing line. If this plant is not tied down, then it will float. Please understand what Riccia is before you buy, as some people think it is a plant that is one piece and gets planted in the substrate, which is not the case.
For more information about this plant, please go to http://en.wikipedia.org/wiki/Riccia_fluitans
To get that terrific lawn look, all you need to do is tie Riccia to mesh with fishing line, thread, or you can use a net ( scrunchies make a great net and are cheap). You then lay the mesh on your substrate with anchors holding it down. Here is a video on tying Riccia to a rock. It is the same procedure for the mesh.
Riccia grows very quickly under high light with CO2, but also does great with moderate light and no CO2.