How do you make custom props for steam workshop?

YellowYeti

First, sorry if this is the wrong forum board to post this, i just thought maybe there were certain programs that i needed or something. But back to the topic, kind of straight forward, i was just wondering how you make custom guns or props and what not in steam to upload for the workshop and what not, or in TF2 how they make the custom weapons a thing. Thanks for any replies.