Depending on the free float tube type/manufacturer, there are a couple of nuances to proper mounting. I've mounted Surefire, KAC, JP and LaRue tubes but each have their own mounting methods. I'd highly recommend the following link from AR15.com that is picture heavy with multiple types of tube mounting instructions:
http://www.ar15.com/forums/topic.html?b=3&f=4&t=226782
It's not a difficult process, but it helps to have the receiver blocks and such. Brownells also has a guntech video that explains tube mounting among other topics:
http://www.brownells.com/.aspx/lid=1...An_AR_15_Video
Good luck with your build.