Once you've installed the MetaMask App, you'll need to connect your MetaMask wallet to the Binance Smart Chain network. After doing so, you'll need to send BNB tokens to the network. Once the network is set up, you can connect to either the Testnet or the Mainnet. To connect to the testnet, you must send BNB tokens to the MetaMask network.
To connect MetaMask to the Binance Smart Chain, click the "Connect Wallet" button. A list of wallets will appear. Select the one you wish to use. Once connected, you can start using the Binance Smart Chain. You can transfer BNB between your Binance wallet and your MetaMask wallet. When you withdraw crypto, make sure you select BNB rather than ETH or Ethereum.