Control access to internal plugin page

Greetings friends!
I was reading a lot of documentation (mostly in English), and although I have an can’t figured out how control access (allow visibility only to registered users) to an internal page plugin.

Specifically I have the following structure:

index.php -> This is the main plugin file. It is the page that you see on the menu and works properly respect for what I need ..
-- process.php -> Page that process forms from index.php. Currently accessible simply knowing the URL.

The idea is that process.php will accessible only for registered users.

I read somewhere that using the conditional tag is_user_logged_in, you can achieve something of what I’m looking for, but I have trouble even include the pluggable.php file and in addition does not show me the php errors. Finally, a real problem.

Conclusion
1 – How do you show content only for registered internal pages of a plugin users?
2 – What is the correct way to include files? Can it be done through absolute URL?
3 – How do you post errors in the above php pages?

If someone could give me a clue at least one of the questions would be very happy. Regards and thanks in advance.