Salesforce Classic:

1. Go to Setup –> Administration Setup –> Data Management –> Sandboxes.

2. Click “Login” link.

(Or)

1. Go to “https://test.salesforce.com/”.

2. Give your username and password as below

If your Production login is example@example.com and Sandbox name is testing, the username should be example@example.com.Testing.
Password is same as Production.