You really ought to have grounds of probable cause for a citizens areest and also if your not a professional police officer you should have back up.
1. HAVE A REASON TO ARREST. Be damn sure you can out lawyer a cop and not rely on "feels" and "muh freedoms". What you do needs to stick to a jury's mind as appropriate.
2. Organize a legally armed possie. It isn't just muscle, it is a validation that this arrest is going down despite the law breaker's protests and brings legitimacy to the scene. If you have a local militia, they may be able to give you advice on how to handle yourself.
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