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.