First you need to go to a modding section and login with your mod.io account (or create one using your email).

When you are logged in, you can log into your account in browser (it will sync mods from browser in the game) Here is a link: mod.io link[mod.io]

In browser go to a selected mod and press "Subscribe"

Next time you open Mod.io in the game it will download the mods you subscribed to (you will have a notification about downloading of mods in the down-left corner)

To Activate Mods, you need to open "Assign Mods" tab in the game and select every mod you want to activate

- Assign Mods
- Join the singleplayer game
- Enjoy installed mods

- Assign Mods
- Join game/create one
- Enjoy the modded content

- If game host doesn't have your mods, you won't have their functionality
- If game host has some mods, but someone that wants to join doesn't, server will not be shown in the player's list of servers
So if you download mods, do not expect a lot of people joining

Marron The Fox has the most up-to-date collection of Missions (Callouts)
They have a distinct preview and good variety of content.
Here is a page of their mods[mod.io]

This mod maker has the most extensive callouts collection.
Each type of the callout has multiple variations (in some cases even 60 variants)
Here is a page of their mods[mod.io]

While they have only two mission mods, both of them have very good quality and are a good addition to the game on their own
Here is a page of their mods[mod.io]