Create a plugin with a process module. Include an init_page(...) function which checks the page request and whether a user is logged in. If not, do a redirect.
Welcome to the Q&A site for Question2Answer.
If you have a question about Q2A, please ask here, in English.
To report a bug, please create a new issue on Github or ask a question here with the bug tag.
If you just want to try Q2A, please use the demo site.