Clogs are part of operating a sickle mower. In addition to what has already been mentioned keep the bar out of/away from what you've already cut. Going over whats already horizontal will just be an instant clog. Sometimes just backing up a little will unclog it. Lifting and lowering the bar repeatedly while backing works sometimes. If you have to get off and clear it lift the bar and SHUT OFF THE PTO at a minimum and better the whole tractor.
Also pen up the dogs and cats if you have any before you start. You can't shut it down faster than Rover can run, at least while he still has all four paws. Yes, that's meant to be a little gory. Better to imagine it now than see it for real while you're mowing.
Used with care, caution, and understanding; the sickle mower can be a pretty handy implement.