Look for a mod on the steam workshop called Play Your Custom Maps. It is probably the simplest set-up of a mod to allow you to use your custom maps.
You download the mod, then copy your map file into the mod, then make a few necessary edits to the mod to allow the mod to use your map instead of the example one provided in the mod.
Long-story-short your map needs to be part of a mod that creates a ruleset which then allows the game to see the map as a valid choice. Play Your Custom Maps adds this ability to the "standard" ruleset and then tells the game what map(s) are valid for the "standard" ruleset.
And you will generally need to go through the Advanced Set-up menu to select the map.