First, correct the premise: Do not breathe underwater. In freediving, you go down on a single breath from start to finish, so any video that teaches you to "exchange air underwater" can be skipped. The "breathing" you actually need to practice happens in the few minutes before you enter the water and after you surface.
Before entry: breathing preparation, not frantic inhaling. What you want is relaxation—slow, steady abdominal breathing for two to three minutes is enough, with the goal of lowering your heart rate. The final breath is a natural inhale, not filling your lungs until they burst: lung over-inflation is neither safe nor will it let you last much longer. There is also an iron rule: Never hyperventilate. It removes carbon dioxide, and carbon dioxide is the very alarm that tells you it is time to breathe.
After surfacing: protocol first, then breathe. The first thing after surfacing is not to take a big breath, but to complete the surface protocol (remove facial equipment, give the OK sign, verbally confirm), then have your buddy continue to observe you for at least 30 seconds. During these seconds, shallow-water blackout (BO) is most likely to occur.
In one sentence: Freediving is about training how to use one breath well, not how to breathe underwater. Remember this, and you will avoid many detours and be less likely to be cheated into extra course purchases.