To begin with you need to register/login to developer.paypla.com
After then you need to create various test accounts.
Then either you can login to https://www.sandbox.paypal.com or by click on ‘ Enter sandbox test site’ above.
There is another unclarity with sandbox is that how to switch from one account to another account.
You might be thinking that by login to sandbox.paypal.com OR by selecting the radio button and clicking the ‘Enter Sandbox’ you will go there then you are wrong. Ideally to switch you need to logout from the previous account.