Each barrel should come with 2 caps, as shown here. They are both threaded differently, and have a completely different look to them.
|Two different types of caps|
The one on the left is, in the context of this design, the "top" cap. The one on the right is the "bottom" cap. It's the bottom cap to which the bib is attached. Note the interior threading on the bottom cap.
First, you'll have to cut away some plastic. You can remove the plastic up to 2 different points, as shown here. In this instruction, though, I'll be removing the lesser example (the one on the right). I've found that this prevents leakage better, and you're less likely to damage the inner threads this way.
Remove the plastic inside the cap as shown here. You can use a utility knife, or something like an Xacto knife. I'm clumsy with large utility knives, so I used an Xacto knife. The plastic is soft enough that not too much effort/strength is required.
Once the plastic is removed, clean up any edges and then screw and tighten the cap into place with your bung wrench. (See What's a bung wrench?)
Prepare the hose bib by applying plumber's/Teflon tape to the threads that will attach to the cap. The ensures there will be no leaks. Remember to apply the tape in the correct direction for threading so it doesn't bunch up. (That part is always trial/error for me, as I can never remember which way to apply it.)
After the cap is secure, then hand tighten the hose bib, being sure to align the spout down.
Replace the barrel and then fill the barrel!