How to make a Mob actually work?

Hello fellow modders!

How make?

Ok, being serious, how do you make a mob? I have attempted to make eight, but none actually function right. I have looked on tutorials on the Wiki, and guides in the forums, but even following them precisely, my mobs never function correctly.

Can someone plese help? Give a detailed tutorial on how to code a mob's AI, because none of the guides i've seen on here go into enough detail. I also cannot get any custom sounds to function properly...