How to import Custom Gun into Shooter Demo

I come from a design and production background, and I have to say that all the beginner tutorials have been excellent so far, I feel like I am really learning.

This is a beginner question, but I have looked it up to no avail… Is there a tutorial or guide in how to replace the Machine gun in the shooter demo with a custom gun? I feel if I do this i will be able to learn a lot from it.