1. Type the following address 'test.salesforce.com' into the URL bar at the top of your web browser. Press 'Enter'.
2. This will bring you to the login screen. Type the email address 'email@example.com.<NameOfSandbox>' and the password. Click the blue 'Log In to Sandbox' button.
3. In Classic, at the top right-hand corner you will notice it says Sandbox: <NameOfSandbox> this means you are logged into Sandbox.
If you are using Lightning, you will see a banner along the top of the screen with the name of the sandbox, in the below screenshot, called 'training'.
If you are a System Admin you can use the following steps to log into Sandbox from your Production Account
1. Go to Setup>Deploy>Sandboxes
2. Click on the Login link next to the name of the Sandbox.
3. Enter the username 'firstname.lastname@example.org.<NameOfSandbox>' and your normal password