1. enter the steam workshop for the masterchief collection.
2. search a mod that you like or explore the mods that are currently on the workshop.
3. when you select a mod you like, press the big green button that says "subscribe", it will start dowloading the mod as a update.
4. Launch mcc from your libraby with anti-cheat disabled.
5. In the gameplay settings, you must change “file share trust level” to “anyone”.
6. (depending on the mod) in case you subscribed to a campaign mod, go to campaign, select the game the mod is for and it should apear as a separate campaign from the normal one. (same with multiplayer)
7. Enter it and select any level you want.

other info:
mods from the workshop don´t overwrite defualt levels so don´t worry if you want to keep it and still play the normal campaign or multiplayer.
to delete the mod just go to the workshop of it and press unsubscribe.