Cabomba is pretty easy, just cut and stick it in the substrate. As long as you don't chop it up too small. I usually cut mine down to about 2-3 inches, and then just stick all the cuttings anywhere I want a new group. as long as it's got enough light it'll jump back up really quick.
I've actually been trying to remove this stuff from one of my tanks and every few weeks I see a new shoot pop up - and that's growing from whatever must still be rooted in the substrate (I'd dig around more but I don't want to disturb what's around it).
The plants on the left are Green Cabomba
The plants on the right are Hornwort
- Ceratophyllum demersum
Both of these plants exhibit aerial roots, a good rule of thumb with these type of stem plants is to usually allow up to 4 inches of stem to be planted into the substrate so that they may develop a good root system at the bottom. If you do not have enough substrate to submerge a 4-inch depth of the stem you could always use weights for the plants which are available at LFS. This way you can get away with clearing only 1-2 inches at the bottom of the stem and still develop nice roots to hold your plants in place. I personally have green cabomba and purple cabomba in my tank right now and these plants are very beutiful and easy to maintain. You will be surprised at how much more beauty you can get out of these plants if you decided to do CO2 injection and fertilizers into your aquarium as well. Given the right parameters these plants will grow really fast. If you decide to clip the plant and replant the stem it is suggested to clip from bottom to top, so that the new growth is what remains. Nothing wrong with clipping from top to bottom and leaving your bottom intact but you will get split plants growing in different direction and not upward most of the time.
Good luck with your plants! Definitely a nice group plant to have in any aquarium.