Simple Method:
1. get the zip file of all my missions (link in the mission description of any of my missions)
2. copy the file ImpasseTW.altis.pbo and put it in your Arma MPMissions directory. It's usually something like 'C:\Program Files (x86)\Steam\steamapps\common\Arma 3\MPMissions'.
3. Rename the file for the map you want: Madrigal: ImpasseTW.OPTRE_Madrigal.pbo

Finding the Arma internal map name takes a little work. If you have CBA_A3 mod, then start up Arma to where you choose the map and hover the cursor over the map name. It will show the name that Arma uses internally.

If you don't have CBA_A3, then choose the map you want on the left, select <<New - 3D Editor>> on the right, and click 'play'. Once the editor loads, click 'Play Scenario' in the lower right. Now open a windows explorer window and navigate to where your missions are stored.
For me it is: 'C:\Users\MY-NAME\Documents\Arma 3 - Other Profiles\MY-ARMA-PROFILE\MPMissions'.
In there you will see a new directory called tempMissionMP.XXXXX where the XXXXX is the map name you want. Use that when renaming the ImpasseTW.Altis.pbo file in step 3 above.

Each time I update the mission, you need to update these copies if you want the changes.


Complex Method:
1. Subscribe to 'Impasse Total War - Altis' using the steam workshop.
2. Create a symbolic link from the steam workshop file into your Arma MPMissions folder (path shown above)
The steam workshop file will be something like:
'C:\Program Files (x86)\Steam\steamapps\workshop\content\107410\3444387114\13781306754930211082_legacy.bin'

So the shell command should look something like this, and may require admin privileges:

mklink &quot;C:\Program Files (x86)\Steam\steamapps\common\Arma 3\MPMissions\ImpasseTW.XXXX.pbo&quot; &quot;C:\Program Files (x86)\Steam\steamapps\workshop\content\107410\3444387114\13781306754930211082_legacy.bin&quot;

Replace the XXXX with the map name (see simple description above for how to get this).
Using this method will automatically update whenever the mission is updated.