You can trade tokens using the Internal Swap feature of MetaMask from your browser extension.
This means you can trade one token to another without navigation to a decentralised exchange. MetaMask will charge a small premium for the service.
To use this feature, select the Swap button from the main screen on your wallet.
You can then select any token which you have in your wallet and the amount you wish to trade. Then select the token you want to exchange it for in the box below.
You will also have the option to select slippage on this screen. Slippage is the percentage difference between the exchange price you are shown and the executed price of the transaction. This exists because in the small amount of time it takes to process your transaction, the price can move. The slippage is the percentage of your movement you will allow. If the price movement exceeds your rate of slippage, the transaction will not go through and you will keep your original token.
When you are happy, select Review Swap and MetaMask will fetch quotes from a selection of exchanges. If there is an exchange which will process the swap, you will be presented with the best exchange rate possible, plus the gas fee.
At this point you must confirm whether you want to go ahead with the transaction.
When you click Swap, MetaMask will now attempt the trade and it will go into processing. All being well, you will shortly get confirmation and the tokens will arrive in your wallet.
Make sure you have added the Custom Token to your wallet so that your new balance is visible.