What is the procedure to claim constructive discharge on an employer who is implementing mistreatment

After completeing my 90 days without any offences, charges, or complaints, a new supervisor stepped in approximately 30 days after and charged me with a final written warning on claims that were not true or investigated by our HR department. No verbal warning, written warning, or corrective actions documented. I went from a great employee to a final written warning of termination. Anything that my current manager deemed unacceptable which is subject to her discression, would result in my immediate termination. The current manager then let me know that this was not personal before handing papers for short term disability because I told her that I have a history a depression and this notice was the last trigger from what I was dealing with. She does know me from our previous employment and we did not have a fond relationship of each other which I do believe resulted in her decision to deliver the final written warning. This is not ethical and or legal. I do not feel comfortable working