Here is a little tutorial that explains how you can use custom weapons in Counter Strike: Global Offensive.
/!\ First of all don’t use this in matchmaking it is not allowed !
Here is my video that explains all of the steps.
Website that i use to get these custom weapons : gamebanana.com
You should only use weapons that are actually available in CSGO.
If you have questions don’t hesitate to write a comment down bellow.