Great question — answering as the UphillTech team:
First of all I would have used plastic pipes and fittings as they're much easier to install. However, as long as you have the metal pieces, you can join them with a "Fernco" rubber joint. It is basically a short rubber hose with hose clamps.
Full answer by F Winker: https://diy.stackexchange.com/a/108847 (CC BY-SA 3.0).
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