Depends on the watch model you own. Mostly you can start an activity of the type Strength, or Cardio (or HIIT, Pilates, and several others). It makes no big difference - it is practically the same data that is being collected, just the screens may vary little bit. Use whatever you feel better with.
Push the lap button, to mark them. I am not familiar with Venu 2 (you'd be better advised on the Venu 2 forum), but on the watches I know, the lap button is the right bottom button, also called Back or Set. In doubts, do not hesitate to consult your user manual, it is always a good starting point, and most topics are covered well.
