* MegEditor-1.3.1 (https://modtools.petrolution.net/tools/MegEditor) and OS Big Editor (http://www.cnclabs.com/downloads/downloader.aspx?id=369) are programs that you can use to open up C&C Remastered files including workshop items.
XCC Mixer 1.47 (https://xhp.xwis.net/) can be used to open and MIX files in the Game.

Once you have subscribed to a Mission/Map, look at its Workshop ID in the Address Bar of Steam. If you can not see this then in Steam you need to click: Steam > Settings > Interface > [Check] Display Steam URL address bar when available.
Example:
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=2120853057
The address for this is ("https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=2120853057")
2120853057 is the Workshop ID for (Nod DEMO Mission 1)

Look in the Steam Workshop folder:C:\Program Files (x86)\Steam\steamapps\workshop\content\1213210\2120853057
1213210 is the Game ID for steam

Use MegEditor-1.3.1 to open the file MAPDATA.PGM
Inside it you should find 4 Files:
MAPDATA.INI
MAPDATA.BIN
MAPDATA.JSON
MAPDATA.TGA

Extract these files and rename them. Example:
Nod_DEMO_Mission_1.INI
Nod_DEMO_Mission_1.BIN
Nod_DEMO_Mission_1.JSON
Nod_DEMO_Mission_1.TGA

*Note: For Multiplayer Maps, all players must have the exact same name for the map for it to work properly. This does not matter for skirmish as you are only playing against the AI.

Now place them in the appropriate Directory in you Documents:
C:\Users\.....\Documents\CnCRemastered\Local_Custom_Maps\Tiberian_Dawn
C:\Users\.....\Documents\CnCRemastered\Local_Custom_Maps\Red_Alert

You can now unsubscribe from the Mission/Map in the steam workshop as the MAPDATA.PGM file is no longer needed.
To remove the Mission/Map from the game, just delete its 4 files from your documents.