Two methods will work:
(1) If you haven't updated to 1.13 yet, simply copy the Diablo II folder in your Program Files (or comparable) directory, then name the second one something convenient. You can then install 1.13 in one directory, rename it, rename the other directory to Diablo II and install PlugY. Whenever you want to switch versions, just change the name of the folder for the install you want to run from to Diablo II and move your save folder to that directory.
(2) If you've already installed 1.13, rename your Diablo II folder in Program Files (or wherever it is), then reinstall the game from scratch (either from the discs or from Blizzard's digital download) and update to 1.12, then install PlugY. You can then change versions by renaming whichever install you wish to use to Diablo II and moving your save folder.
There are of course other ways to do this, but those methods will both work (although I will admit that they require a bit of disk space, D2X takes up a little under 2 GB and so will the second install).
The reason you need to rename the folders is because PlugY doesn't know where to look for the rest of the game files unless it's in the Diablo II folder. If you want to avoid having to rename the files in order to change versions, you can follow this process:
(1) Install 1.13
(2) Rename the Diablo II folder to something else, say "Diablo II - v1_13"
(3) From the new location, create a shortcut to the Diablo II executable and put it someplace convenient; name the shortcut such that you'll remember it's for 1.13
(4) Install Diablo II again and update to 1.12 (NOT 1.13)
(5) Install PlugY
(6) Create a shortcut to PlugY.exe in a convenient place
(7) Move your save folder to the Diablo II folder (the 1.12 one, the folder named Diablo II)
The reason you do that is because both versions of Diablo II will look for the save folder in the Diablo II directory (although you can probably change that if you want to, I think it might involve editing registry files and I don't want to get into that), but you can run 1.13 just fine from the renamed folder (PlugY can't, it will look for the game in Diablo II).
Edited by Explopyro, 03 August 2010 - 10:52 PM.