Well, believe it or not, it all starts from an idea. Had an idea where a certain vehicle might be fun to use in a certain situation? Perhaps and idea where players have a certain loadout that you had in mind? Or maybe you watched a cool movie and wanted to replicate it in Arma? All those ideas can inspire you to create a cool mission that our players can play through, if executed properly.

A mission can be defined as a goal that can be achieved by completing some goal-related tasks. A task can be defined as an activity that, once completed, contributes to achieving a goal. There can be multiple goals, where each goal has multiple tasks.
So when designing a mission, you need to think of a goal, what do you want players to achieve? For example, a goal would be: Rescue VIP - rescue a Very Important Person, who is currently being held by enemy forces somewhere in their territory. This goal can be achieved by completing some tasks, like collecting intel on VIP's current position and attempting a rescue, collecting intel on number of enemies guarding the VIP, capturing enemy HVT that can be used as a bargaining chip, etc. All of these goals can be defined by the mission maker so that the player can have multiple options when approaching a problem.
Avoid designing linear missions, while they can be nice from a grunt's perspective, Squad or Platoon leaders will most likely struggle, as they will see no point in their existence. Make sure Platoon or Squad leaders have some freedom of choice when it comes to mission's execution.
To continue a bit of theory, you will need to understand the term Player focus. Player focus determines how much disorder there will be in your mission, so more player focus equals to less disorder and vice versa. Generally speaking, the further you progress in your mission, the less focus the players have, as shown in the image below (the intensity of focus can vary depending on mission quality, which includes performance, immersion, leadership, etc.).

To skip the boring theory, lets jump to some technicalities of mission making.
The mission directory (folder where the mission is saved) is by default located in: C:\Users\YOUR USERNAME\Documents\Arma 3 - Other Profiles\YOUR_USERNAME\mpmissions\.
When you open your editor and decide what map you want to make your mission on, you have the option to save the mission. To save a mission, do the following:
When you save the mission a new folder will be created in mission directory and will look like this: FU_Name_Of_Mission.map_name.
Example of various mission folders located in mpmissions, containing missions that have been played through, WIP missions and mission ideas:

Avoid having spaces in mission name, as they will not be displayed, instead of them
%20sign will be displayed, which is quite annoying. Use_or-sign instead.
It is advised to use some sort of a tag before the name of the mission, so you know for which community it was made for, if you play for more than 1 community. For FU, your mission name would then have the
FU_tag.
Do not delete the
.map_namefrom the mission folder, since that is how the game knows on which map your mission is located.
The editor consists of 4 parts:

The menu bar consists of 8 sub-menus: