1. if the billet is oversize, just touch of roughly from the relevant sides. Touching of can be as simple as with the cutter spinning, just jogging up until the cutter marks the material, then hitting the relevant zero button (or entering size if not working of zero)

2. Depends on tools at hand, and how many you'll be doing.
For any more than a few, you really need to make up some kind of jig capable of accurate mounting.
However for one offs, you really need some kind of additional tool. For cnc, a touch probe is the best suited, however there's nothing stopping you from using old manual techniques, such as edgefinders, dial gauges, bit of paper stuck on, and jogging up to where you need to be.