Cut off an inch of rope and have a cutting board and sharp, large knife at the ready. Begin by cutting the inch into quarters. When that is done begin mincing.
The danger is mincing the rope down too small is that when you load the pipe these small pieces are sucked into the airway and clog it. I usually leave about 20% in 1/4" X 1/4" pieces; I mince the rest down to about a quarter of the size of the chunks.
If the finished product doesn't smoke well you can always mince more; on the other hand if you've minced too far you're stuck with it.
It takes some practice to know how to do this, so start with no more than 1" of the rope; in this way you'll have the luxury of repeating the process and applying what you've learned. Before you know it you'll be mincing 100 g (or more) at a time.
BTW, are you preparing SG or GH rope? Gawith rope is mild, GH a powder keg. If the latter and your body isn't used to HO tobacco, smoke very slowly and listen to what your body is telling you. If your head begins to sweat or spin, put that pipe down.
Also, GH Brown Irish X is the strongest rope out there; their Black Irish X isn't far behind. The flavor of the Brown Irish X is mild and to my tastes a 50/50 mix of it and GH Brown Happy Bogie is far more acceptable as regards taste.