Between now and April is the ideal time to divide ornamental grasses,depending upon where you live. But the key is to get them divided before they start growing for the season. Just dig up the clump and cut ,chop or saw the clump into small pieces. There really is no exact science about how to go about this.And...if your ornamental grass has been growing in your landscape for a while, I suggest you eat your Wheaties, because you will have your work cut out for you. Older, established clumps of ornamental grass will come out of the ground in one big round clump like a bowling ball. And trying to divide that clump is going to be about as easy as dividing up a bowling ball. I'm not kidding, but I know you can do it. The first time I tried to divide one I think I had every tool on the truck out there trying to get that thing apart. Eventually it worked, piece by piece. But I'm going to share a secret with you. Something I learned the after the first futile attempt. Sawzall, or more correctly, reciprocating saw. They now make pruning blades for reciprocating saws and they work great for dividing up clumps of ornamental grass.