How to setup vO2max measurements on Apple Watch

There is no setup as such;

Tighten the watch band a touch and clean the sensors on the bottom of the watch..

Apple says;

Your Apple Watch gives you a cardio fitness ESTIMATE by measuring how hard your heart is working during an outdoor walk, run or hike.VO2 max relates to maximal exertion. So you’ll need to log a pretty intensive workout to generate a reading. And you’ll need to sustain it for at least 20 minutes.