How do I write a good "enemies to lovers" side-plot?

Side plots are just like the main plots only they happen in the times between the action of the main plotline.

Enemies to lovers is a very old plot and not an uncommon plot in real life. Just remember it’s a side plot, not the main plotline. I had one side plot which was much like what you’ve described and the way the relationship came to light was the secondary character and the Main character were talking over some ideas on how to cross over into another land without notice and while talking the secondary fellow reached in his pocket and seemed surprised and pulled out his arch rivals undies which naturally everyone saw, but no one recognises them. He stuffs then back down in his pocket rather quickly and then the main character looks at him and says, “Ok, so, are you taking them back to her or are you planning to wear them on special occasions?”

A good side plot can be a wonderful thing if you put a little time into it. Just like you did on your main plot, only it’s a cute side note.