So, you have some ETH and want to convert it to Bitcoin (BTC). The "easy" way seems obvious: send it to your Coinbase or Binance account, make the trade, and withdraw the BTC. But this "easy" way comes with a high cost...
https://blog.b2p-central.com/How-to-Swap-ETH-for-BTC-Privately-Without-Using-a-Centralized-Exchange