How to make an actor (NPC) follow a spline?

I’m making an aquatic ambient where there are some fish which follow a path (spline) but I just dont know where start, I can’t find a good tutorial to do that. so that’s why I’m asking your help. If you know somthing that can help me, I’ll be very grateful. I find a plug in iTween but is buged and is not possible to compile so is impossible to build the game.

Although iTween makes it very easy, I think is better do it without plugin.