new admin panel without plugin

I want to implement a form which connects to the database and sends and updates a table into the Users Tab in the Admin Panel. I know for instance how I can do this with a normal page, make a page template etc. but I have no idea how to do this with the panel.

I read how to add a new “panel” into the admin and use a function but that didn’t really eplain how to put the content. My page code will be complex with php of its own.

Any ideas anyone?