1. Download the mod which you wish to ruin and 1 credit 1 second core

2. Navigate to the Empire at War workshop folder C:\Program Files (x86)\Steam\steamapps\workshop\content\32470

3. Open the folder for the mod which you wish to ruin. The name of the folder will be the steam worshop ID. For example, thrawns revenge would be 1125571106

4. Open the folder Data\XML\Units folder, there may also be more folders with upgrades, heroes, or other units which you will also need to open depending on the mod.

5. Copy those files into another folder with the same file structure which will act as your content folder. By same structure, I mean use the same folder names and put files into a folder with identical names to the source mod.

6. Open every XML file containing units, heroes, upgrades, buildings, and other construables, and change the given variables using find and replace. (make sure that find and replace is set to Regular expression, and . matches newline is unchecked)

• Find:
  (<Tactical_Build_Time_Seconds>\s*[^0].*</Tactical_Build_Time_Seconds>)|(<Tactical_Build_Cost_Multiplayer>\s*[^0].*</Tactical_Build_Cost_Multiplayer>)|(<Build_Cost_Credits>\s*[^0].*</Build_Cost_Credits>)|(<Build_Time_Seconds>\s*[^0].*</Build_Time_Seconds>)|(<Tactical_Build_Cost_Campaign>\s*[^0].*</Tactical_Build_Cost_Campaign>)|(<Absolute_Income_Modifier>-.*</Absolute_Income_Modifier>)
  
• Replace:
  (?1<Tactical_Build_Time_Seconds>1</Tactical_Build_Time_Seconds>)(?2<Tactical_Build_Cost_Multiplayer>1</Tactical_Build_Cost_Multiplayer>)(?3<Build_Cost_Credits>1</Build_Cost_Credits>)(?4<Build_Time_Seconds>1</Build_Time_Seconds>)(?5<Tactical_Build_Cost_Campaign>1</Tactical_Build_Cost_Campaign>)(?6<Absolute_Income_Modifier>0</Absolute_Income_Modifier>)

• Find: <Tactical_Build_Time_Seconds>\s*[^0].*</Tactical_Build_Time_Seconds>
  
• Replace: <Tactical_Build_Time_Seconds>1</Tactical_Build_Time_Seconds>
  
  
• Find: <Tactical_Build_Cost_Multiplayer>\s*[^0].*</Tactical_Build_Cost_Multiplayer>
  
• Replace: <Tactical_Build_Cost_Multiplayer>1</Tactical_Build_Cost_Multiplayer>
  
  
• Find: <Build_Cost_Credits>\s*[^0].*</Build_Cost_Credits>
  
• Replace: <Build_Cost_Credits>1</Build_Cost_Credits>
  
  
• Find: <Build_Time_Seconds>\s*[^0].*</Build_Time_Seconds>
  
• Replace: <Build_Time_Seconds>1</Build_Time_Seconds>
  
  
• Find: <Tactical_Build_Cost_Campaign>\s*[^0].*</Tactical_Build_Cost_Campaign>
  
• Replace: <Tactical_Build_Cost_Campaign>1</Tactical_Build_Cost_Campaign>

• Find:
  (<Population_Value>.*</Population_Value>)|(<Space_Override_Population_Value>.*</Space_Override_Population_Value>)
  
• Replace:
  (?1<Population_Value>0</Population_Value>)(?2<Space_Override_Population_Value>0</Space_Override_Population_Value>)

• Find: <Population_Value>.*</Population_Value>
  
• Replace: <Population_Value>0</Population_Value>
  
  
• Find: <Space_Override_Population_Value>.*</Space_Override_Population_Value>
  
• Replace: <Space_Override_Population_Value>0</Space_Override_Population_Value>

If you wish to remove building upkeep, use this as well. (Thanks to CometRed06S for finding this one)

• Find: <Absolute_Income_Modifier>-.*</Absolute_Income_Modifier>
  
• Replace: <Absolute_Income_Modifier>0</Absolute_Income_Modifier>

• Find:
  (<Tactical_Build_Time_Seconds>\s*[^0].*</Tactical_Build_Time_Seconds>)|(<Tactical_Build_Cost_Multiplayer>\s*[^0].*</Tactical_Build_Cost_Multiplayer>)|(<Build_Cost_Credits>\s*[^0].*</Build_Cost_Credits>)|(<Build_Time_Seconds>\s*[^0].*</Build_Time_Seconds>)|(<Tactical_Build_Cost_Campaign>\s*[^0].*</Tactical_Build_Cost_Campaign>)|(<Absolute_Income_Modifier>-.*</Absolute_Income_Modifier>)|(<Population_Value>.*</Population_Value>)|(<Space_Override_Population_Value>.*</Space_Override_Population_Value>)
  
• Replace:
  (?1<Tactical_Build_Time_Seconds>1</Tactical_Build_Time_Seconds>)(?2<Tactical_Build_Cost_Multiplayer>1</Tactical_Build_Cost_Multiplayer>)(?3<Build_Cost_Credits>1</Build_Cost_Credits>)(?4<Build_Time_Seconds>1</Build_Time_Seconds>)(?5<Tactical_Build_Cost_Campaign>1</Tactical_Build_Cost_Campaign>)(?6<Absolute_Income_Modifier>0</Absolute_Income_Modifier>)(?7<Population_Value>0</Population_Value>)(?8<Space_Override_Population_Value>0</Space_Override_Population_Value>)

In Scripts\Library\eawx-mod-fotr\gameobjects or Scripts\Library\eawx-mod-icw\gameobjects

• Find: Ship_Crew_Requirement = [0-9]+
  
• Replace: Ship_Crew_Requirement = 0

If it isn't working, go to your steamapps/workshop/content/32470 folder and check to see if the file for your mod is the same as the one you uploaded, as it may not immediately update. You can also try checking you file structure to make sure it matches that of the original mod. Then, check your json file and make sure that it is formatted correctly, by comparing it that of a working mod.
Give your content folder the same name as your mod's workshop ID.

Tag your submod with "Unit Cap" and/or "1C1S" when uploading it.

To make the mod player only, make a separate version of your submod for each faction by modifying only the files/units belonging to a single faction. If there are multiple factions that use the same variant of a unit or upgrade, try creating an identical variant of the ship (examine the original Thrawn's Revenge files for an example of how this is done) and replace all references to the original with the new ship in faction specific lists (ie. <Tactical_Buildable_Objects_Multiplayer>
in Skirmish_starbases.xml), and add it to lists which are not different between factions (ie. <Bombardment_Required_Orbital_Ships> in Factions.xml).

Don't forget to include a modinfo.json file

If anyone figures out how to fix the stability issues when running 1C1S and infinite unit cap combined, please let me know.

Notepad++
https://notepad-plus-plus.org/

EAW Workshop Uploader
https://www.petroglyphgames.com/eawmodtool/

Low effort video if you need it.
https://youtu.be/BH2v7QKlDyE