What's the best way to tune a guitar? Should I use a Chromatic Tuner?

I think it is best to learn to do it by ear. It is good training for your ear and with practice you'll be able to do it quickly even when you don't have a tuner with you. You can carry a pitch pipe with you to get the first string and then tune off of that. It's more important for your strings to be in tune with each other than to try to be perfectly on the right pitch, unless you are playing with a piano. And in that case you are better off tuning to the piano, which might be off and not on the right pitch anyway. Yeah, you can go flat and tune up to it. That works well.