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...

Ideally you should see a big list when you mouse-over profile menu-item. Or  you see a big list when you click profile menu-item But in some case you see like this   The reason of this variation is that your account type is...

I realized that there is no query string at all. paypal simply forward to the redirect URL. I started using sandbox. I made sure that auto return is enabled. and also PDT is enabled. still I am redirected to a plain URL. finally I read this...