I pounded them together with a mallet. You can put a piece of scrap wood underneath the PVC pipe so you’re not pounding it directly on a hard surface like concrete. This was much easier for me as I couldn’t do it by hand alone.
One more thing: I saw mine was leaking a little bit at the coupling when the pump is running. It’s not a big deal as it’s leaking into the growing medium of the second quad pot. However, would the leak get more pronounced, I think going to the local hardware store and buying a 10-ft section is cheap and would make the whole setup hassle-free.