Its pretty straight forward. Pull a plant out of your substrate. I wiggle it while pulling gently. Blyxa roots are usually thick and plentiful. This way you will loosen the root mass and get them to pull out easy with most of the roots attached.
After you have removed a patch look at the plant. It is a stem plant that branches. In the clump that you pulled out you will notice 2,3,4, or more plants actually form that one little bush.
Now simply snip them apart trying to save as much of the stem below the leaves on each one. This will make them easier to plant. The new ones you have cut will not have roots but will establish them after you plant them. Saving as much of the stem below the leaves will make them easier to plant. Shorter ones will want to pull out to of the substrate in a strong flow.
If the stem is short below the leaves and you think you'll have a hard time getting it to stay in the substrate simply pull some leaves from the bottom of the plant to make more of the stem available to shove deeper into the substrate.
I trim my roots back leaving an inch or so on the plants with the roots before replanting.
After a good trim and replant they will take a little while to take off again. But as soon as they start growing...wow a real weed.