I have about 195k miles. My engine light came on and I assumed it was alerting me that a cylinder or two might need new spark plugs. Well, I made the mistake of not scanning the error and just changed all 6 spark plugs. I disconnected the negative terminal before I changed the spark plugs. After the change, the engine light returned, so I scanned. Three cylinders came out as bad (this is the only info offered from auto parts technician). With that, I again disconnected negative terminal and replaced all COPs. Engine light once again returned. Rescanned, this time it was only cylinder #5. (If I'm correct, #5 is nearest driver, or if standing in front of engine bay, it is located at right rear. Please correct me if I'm wrong.) I pulled COP #5 and made certain I had it seated properly, and made sure the wire connector clicked onto the COP. Engine light still on. Scanned again, Now it's reading P0305 Cylinder 5 Misfire Detected. Secondary: P0205 Fuel Injector 5 circuit/open, and P219A Bank 1 Air-Fuel Ratio Imbalance. Given that both error codes are for #5, I'm now assuming I must have damaged the wire harness to #5.
My question is, how can I troubleshoot this and repair? I have a multimeter but honestly, I don't have a lot of experience using one.

Having just looked this up, I can at least confirm are correct on which one is #5.