Pretty much Kelp! (I garden outside too, and I gotta say aquarium plants are FAR easier to set root!!!)
Just snip the plant off, maybe half way as radar suggests, but at least try to give yourself a 15cm cutting. Then on the lowest node or two (thats where the leaves grow out of the stem) strip the leaves off. The new roots will grow from those nodes. Then insert the plant into the substrate so those nodes are covered. And]y leaves that you bury into the substrate still joined to the stem will just rot.
Its always helpful to use a nice pair of sharp little scissors, but snapping it can work just as well.
Nero