A short training video can reveal details that are difficult to notice during a set. It can also distract you if recording becomes the main event. Decide what you want to observe before placing the phone. One clear question about a familiar movement is more useful than collecting a camera roll of workouts you never review.
Follow your gym's filming rules and keep other people out of the frame. Place the device where it will not obstruct a walkway or create a trip hazard. Use a stable position and an angle that shows the relevant movement. Do not balance a phone on equipment that someone else needs or interrupt another person's set to improve your shot.
Review the clip after the set rather than between repetitions. Notice whether your setup remained consistent and whether the movement changed as you became tired. Video has limits: a single angle does not explain every sensation or diagnose a problem. If you need technical guidance, a qualified coach can interpret the footage with information about your goals and experience.
Save only the clips that serve a purpose, with an exercise name and date. Compare similar views when reviewing later sessions. A useful recording leads to a manageable observation, not a long list of imagined flaws. Keep the next action simple enough to practice, then put the phone away and return your attention to the actual training.