Normally, this is done by cutting into the copper trace with a sharp cutter blade until it is through at the location where you want to separate the trace. You should check that the traces are isolated with a multimeter to be sure afterwards, though…
While I don’t use these much myself anymore for prototyping, when I did I used a Dremel tool and routing bit intended for plastics to tear off copper where I needed a gap. Too much time with a knife trying to electrically separate things caused this behavior…
As with everything YMMV, but some care with the Dremel proved much less frustrating than using a knife for me.