[stephen_g]

It mostly just came with practice but perhaps I already had some advantage because I've done a lot of singing and also played wind instruments (trumpet mainly) for many years. Getting headaches doesn't sound great though - perhaps you were pushing too hard? You want to push through discomfort (and there is a lot of that especially at the start) but not into pain. Maybe just start with four or five laps at a slow pace and then try to add another lap every two weeks or so?

In terms of videos I quite like this channel on YouTube - https://www.youtube.com/@SkillsNT - it's all free content (they make some money with paid intensive camps every now and then at various places around the world but obviously you don't need to do that!). They have a fair bit of stuff about breathing, including exercises to do both in and out of the pool to improve breathing and how your body responds to CO2 in the bloodstream. Those exercises could also potentially help.