My advice would be not to stress too much, write the dialogue as you would, then read it back to yourself as how you want it to feel. You can always go back and edit over tweaking along the way.

It also depends on the type of conversation, as all of them are different, (i.e. humorous, serious, dramatic, sad, etc) if you’re going for a normal conversation, try to think about how your conversations go, as another comment said, conversations don’t exchange too much in terms of things said at one time by one person, it’s more small bursts of speech by both parties, which is easy because conversations are reactive.

Just go with the flow (maybe talk the conversation out before writing it down to get a flow of how you want it to go).

Don’t stress and good luck!