It's very easy. you can follow Facebook's own instructions...
here:
http://developers.facebook.com/docs/...e/plugins/like
There's also at least 2 Facebook Like mods for 3.8 (that would work on any 3.x version) in the mods section- although they instruct you to use the showthread or postbit template so you can like the thread or post rather than the whole forum, either can instead be used in the header. Plus the official Facebook site gives instructions so you can specify the URL shared by the Like button, you can make it your forum homepage if you prefer.