The best guide for what you want is here:
http://mybbhacks.zingaburga.com/showthread.php?tid=293
Just follow what the setup tells you.
What you need to understand is that XThreads is a tool that you can use - it does not do anything for you.
You can make threadfields in the custom threadfields. When you make a threadfield you can select which forum it can be used in. Then, when you make templates, you can set a prefix for that forum you have chosen when you made a threadfield and then create templates using that prefix in global templates.
Inside the templates you have made for that forum you can add the variables from the custom threadfields and they will show in the forum.
That is it