How To Mod Horizon
A basic tutorial on how to mod Horizon can be found here
You also have the ability to create missions or to disable official ones. Instructions on how to accomplish this task can be found here
As with any mod, be sure and carefully follow the instructions and make backups of stable versions so that you don't lose any work.
If you run into any problems just make a post in this section of the forum and someone will get back with you as soon as possible.
Happy Modding