I can’t stress enough how incredibly buggy the Creation Kit is. Again, there are plenty of tutorials online on how to do that as well. This tutorial also assumes you have placed your NPC somewhere on the map (i.e an interior or exterior cell). If you need to know how to do that, there are loads of tutorials, both written and videos, online that can show you how to make NPCs from scratch.
Also, this tutorial is going to assume you already have an NPC made in the Creation Kit for you to convert into a follower (which means you already have a mod you want to edit and have set as an active file).
I will warn you now that it is a lengthy process, but if you take your time and hang with me step by step, you’ll get it down. This tutorial is exclusively for the trickier stuff like how to get a custom follower to say custom lines to you instead of the default options the vanilla followers say and it will show you how to get your DB to say custom lines to your follower when it comes to giving them commands. I will not be covering the actual “how to add” audio stuff since frankly, when compared to what I will cover here, that is the easy part. In this tutorial, I am going to show you how to make custom voiced followers.