What advice would you give for someone to get started with freelancing/side projects to earn some extra money?

Get a contract, T&M contract.

Well, I wish I don't have to do that. I'm a full-time developer but the pay is quite low where I'm based in, and it looks it's the only way for me to get some extra income..

As many others already said make a contract.

I thought one fcking time, naw this client seems cool. Yeah ... still "waiting" for the money after years. Almost needed a lawyer at the end.

And most important make a clear definition what you will do for what amount of money. And if possible let your customer make a deposit.