Pawing is one of the easier exercises when training a cat.
Pick up a treat and squat in front of the cat so that you are at the same height. Tap her paw gently with your finger and give a short command ("shake", "give" or similar). Don't forget to use the clicker as soon as the cat gives you its paw. After the click, praise the cat lavishly and give it a reward.
Once she has understood it, it is only a matter of time and practice until she listens to the command directly and lets her paw be shaken.
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