How do I start playing competitively?

So I just came back to pokemon around 2 weeks ago, I've been having a lot of fun, and have been watching tons of wolfeyvgc and all of the stuff i see is really cool and I want to try, I'm not great in terms of my pokemon knowledge but I know a good amount, how do I practice double battles? Because I don't know of something in game that doesn't require friends(I play scarlet and sword) so please I'd like any kind of advice you can give.

Also if yall got any teams for flygpn pr garchop please do tell because they're my favs