Click one of the "Connect wallet" buttons, click MetaMask, and connect your desired wallet in the extension popup.
Filter pools by desired assets. In this example, we'll use WETH and BAL
Click on the desired pool to navigate to the pool management page
You can choose to invest one token, a subset of tokens, or all of them. The pool will calculate how many pool tokens (BPTs) to give you for your input tokens. If you join the pool according to pool weights ("Best price"), you will not create any price impact. If you join in an unbalanced way ("Custom amounts") you may impact the price.
To invest a single asset, choose "Custom amounts".
You can choose to withdraw a subset of tokens or all of them. If you decide to withdraw only one toke you can withdraw up to the whole value of your investment. In that case, the app will automatically swap other tokens to the one you're withdrawing. Eg it will sell the WETH in that pool and buy the BAL you're requesting. To withdraw a single asset, choose "Custom amounts".