Typically what vanark said. Slide it over and tighten the screw down on the extension. However...
If anybody knows of a long stud I would like to know. My Lowel extension bites the threads, I ended up shearing it off at the base of the threads one night from mic weight. I bought two other sizes and it still bites threads. My only temporary solution was to bite the 3/4 end and buy adapters for the 1/4 end if I need to use the stud to T-bar. I'm thinking of using a long bolt with the head and threads cut off, seems like it would slip though.