OK, let's say you have WOE installed in c\Thirdwire\Wings Over Europe and want to create a WW2 European theatre install:-
1) Navigate to the Thirdwire folder, right click on the Wings Over Europe folder and 'copy', then move your mouse to a free space in the Thirdwire folder, right click and 'paste'
2) Next rename your new Wings Over Europe folder to something like Wings Over Europe WW2ETO, go into the folder and send the WOE.exe to the desktop as a shortcut to that install, and rename the shortcut accordingly so you don't get mixed up on your desktop. You could also change the icon to something more appropriate too.
3) Now the fun bit. In your new Wings Over Europe WW2ETO folder you'll need to go into the objects\aircraft and objects\ground objects folders and delete all the modern stuff and doenload and install the WW2 planes, tanks, ships etc you want, together with the appropriate skins, pilots etc.
4) A WW2 weapons pack should round things off nicely.
5) You can make as many installs as you like, for WW2 Pacific or Med. action, or any other theatre or era that we have mods available for.
For a WW1 install it's even simpler - buy First Eagles!