I asked Medtronic this once. I was already on the phone with them because a sensor had failed early due to two failed calibrations (sigh), so while I had them on the line, I asked.
They said to put the test plug in to seal the hole (otherwise it’s not water resistant). Then to use warm soapy water or alcohol pads.
I don’t do that very often. I use my fingernails after every sensor change to get as much as I can off, then use alcohol pads to get some more off after a couple of sensors. About once a quarter, give or take, I use warm soapy water.
Basically, I don’t try to keep it perfectly clean, as it’s a lost cause.
I don’t like using the test plug, since it tends to make my pump think a new sensor has been inserted. So I turn off the sensor in the pump’s settings, THEN do the cleaning, then turn the sensor back on after taking the plug out. It’s annoying, which is why I don’t do it often.