How do I purchase Apex coins with Steam?

Hey there, I'm wondering if you could help me out with something. I'm a fan of Apex Legends and I've been wanting to purchase some Apex coins to get some in-game items, but I'm not sure how to do it through Steam. Could you walk me through the process step-by-step? I've looked around online but haven't found a clear explanation yet. Thanks in advance for your help!