Spot metering is one way to do it, but you might find it's actually slower than using full manual once you get enough experience in a particular area (for example backlit stage shows). In full manual, you'll set your exposure and adjust on the fly in a single click as the light changes. Spot metering is slower because you have to find a decent spot to meter, then lock exposure, then go on to actually focus, frame, etc. It takes an extra few seconds and 2+ movements just to spot meter where adjusting your ISO is a single movement is much quicker (and can be done while you are framing for example).