First have an idea of what you want to create with the mod, For this example I will do Order 66 on Coruscant Balconies.
Step 1: Open the Game
Step 2: Click/tap Single player
Step 3: Click/tap Sandbox Editor
Step 4: Select the map from the drop down "Map" Optional: Select a custom map from the drop down "Type"
Step 5: Add the AI and teams that you want the AI to be on
Step 6: Start the Game and Immediately pause the game with the actions to the right of your screen
Step 7: Select the "Delete Units" Option to the right and delete all the bases on the map
Step 8: Place buildings for each AI and use the option at the top of the right side menu to change the AI you are placing the buildings for(dont forget youself)
Step 9: Click/tap the settings cog and click/tap save and Ok
Step 10: When selecting the save on android tap Single player and load saved game and select the one named what you named it, For PC do the same