How do I convert ETH to BTC on Coinbase?

I'm a bit of a crypto novice and I'm trying to figure out how to convert my ETH holdings into BTC on Coinbase. Could you walk me through the process step-by-step? I've seen mentions of trading pairs and market orders, but I'm not quite sure how they fit into the picture. Additionally, I'm concerned about the fees associated with the conversion and whether there's a way to minimize them. Lastly, I'd like to know if there's a specific time or market condition that would be ideal for making such a conversion. Thank you for your help!