Assuming you are trying to connect the tubing to 2L bottle cap, you can pick up a barb to pipe thread fitting (see pic) to fit your tubing, at any home improvement or hardware store. Then, drill a hole in the cap a little smaller than the threaded side of the fitting. Now you can wrap the thread with the Teflon, as described above, and then thread the fitting into the hole. With a little force, it will form threads into the plastic cap. Now you should be able to just slide the tubing over the barbed side, and as long as you have the correct size to fit your tubing, you should be leak free.