The Merge will happen in a few hours only. You can leave your ETH on a centralized exchange, waiting for the ETH POW listing, but it will take a few weeks at least. If you want to be part of the game, you can take the risk to leave your ETH on Ethereum mainnet through your Metamask wallet and wait for the ETH POW fork (24 hrs after the Merge) to get free ETH POW coins. In that case, please follow the instructions below to avoid replay attacks:

After the Merge, send your assets (except ETHW) and ETH on Ethereum PoS from your main wallet to another secondary wallet. After this, you can send your ETHW to sell to Poloniex (or any supported exchange). This way, if a bot (or someone) tries to carry out a replay attack on PoS, the transaction will fail since you already moved your ETH to a different wallet.

That's all you need to do. 

Disclaimer: I do not work for Poloniex, so you can use other exchanges supporting ETH POW like OKX or Bitmex if you wish. Happy ETH POW fork!