To deploy to a sandbox, be sure that you have already created a sandbox, or head to the Creating a sandbox section of our documentation for more information.
Follow the steps in Creating a Deployment to start a deployment. At the second step, Select network and connect wallet, you'll see the name of your available sandboxes in the Destination Network dropdown.
The Deployment Account section that appears after selecting a sandbox allows you to specify which of the 10 auto-generated accounts to use for the deployment. By selecting one of these accounts, all transactions will automatically be confirmed using the specified account. You can optionally select MetaMask to confirm your own transactions through MetaMask like you would for any other network.