Making Mod Multiplayer(Server) Compatible

I just uploaded my first mod to Curse today, and already I have a few downloads(woot!). However, one person commented that they tried to use the mod on their server but it won't work since it is made for client side only. I didn't think of this, even though I was intending to use it on my own server. I assumed it would be server compatible. :/

I tried googling this and couldn't find any answers. So, how do you make a mod for server side?