u/Ghostly_Gossip6

In a few months, my fiancé will be coming to the US from Europe and I will be going back with him. The problem is I'm unsure how to make sure we get on the same flight back to the EU. What would be the best way to book the flight to ensure we are on the same plane and sitting next to each other? I had thought of booking at the same time but I was wondering if there's more of an efficient way to do it to minimize making a mistake.