How do I buy platinum on Steam?

I'm curious, how does one actually go about purchasing platinum on Steam? I've been meaning to get into some of the more advanced features and games that require it, but I'm not sure where to start. Do I need to LINK my bank account or credit card? Are there any fees associated with the transaction? And perhaps most importantly, how secure is the process? I'd love to hear your thoughts on the best way to navigate this process.