I'm a new user and type 2 (no meds) I have used 3 sensors so far, so not an expert.
It believe it is best to attach it but not activate it for a couple of days, and I find after activating it I need to ignore the first few hours of readings as they can be ridiculous. Also the last few hours can be erratic.
It is wise to attach it on the under arm in the fleshy part. Hold your arm out straight, palm down. Grab the skin between arm pit and elbow and attach it there. That works best for me.
I find the readings are consistent - ie. once it has settled down there are no rogue readings. You can rely on the trends and variances, if not the levels.
Each sensor is different and requires calibration with your finger pricker in the beginning to determine how much, on average, they differ, using times when you should be normal and times when you should be high. Some sensors read lower, some read higher, some are almost spot on. Out of my 3 I've had 2 lower and 1 spot on. When calibrating remember the sensor may be somewhere around 10 to 15 minutes behind your finger pricker, but not always!.