How do I create a simple form and use prefixes?

[ZiNgA BuRgA]

You need a strong grasp of how the MyBB template system works.  If you don't, XThreads will be very painful to use.

The basic idea is to create the custom fields (should be able to find it in the Admin CP), then remove the post input box from the relevant templates.  But since templates are usually global, removing it from the templates will affect the entire forum.  To avoid this situation, you duplicate the template and point one forum to use this custom template.

I suggest working through some of the examples.  It'll help you get an idea of how it all works.