I believe the proper place to post questions about UAT is under 'Functional Testing' forum, but I'm not exactly sure? This is forum is for Usability and Accessibility Testing which is very different from UAT.
Maybe there needs to be a new UAT forum?
But to start answering your question. UA Testing should be validating the initial requirements that were gathered at the beginning of the project as to what functionality the application should have. The Users need to validate that the system has the functionality that they expected.
It's good to get commitment from the Users as earliest as possible as to who will be performing the UA Testing. Definitely have documentation for them to follow, record their results, track defects and then when the users feel the application has all the expected requirements and is ready for production, have them "sign-off" that their UAT is complete.