Wario World Multiplayer (Alpha)

Not him, but it's actually pretty easy to do.

First, you need to have Wario World visible in your games library in Dolphin, via making sure the Paths section of Config is pointing to where the ISO is. Then, right-click it, pick "Properties", and look through the tabs on the top of the window that pops up and find "Filesystem". From there, right click "Disc - GWWE01" and pick "Extract Entire Disc", and have it put it somewhere you can access easily.

Next, go into the folder this made with the game's unpacked files and look for the "main.dol", should be pretty visible and easy to find. Overwrite that file with the one from this page, and now the game is modded. Don't move or mess with anything else, just leave it all as-is besides that .dol file.

Last bit you need to do is repack it, which is also very easy. Either stick the folder full of the unpacked files that Dolphin made into the folder where you've already told it to find your games, or add the path for the folder manually to its list. You should see another copy of Wario World pop up, you'll be able to tell it's the unpacked one by looking at things like filesize, although you can also make it easier by just temporarily moving the original Wario World ISO somewhere else instead. Right-click this new Wario World, and hit "Convert File", then hit OK with the default settings, which will pack it into an ISO file.

Let it finish that up, and you're all done, with a crisp new ISO that is modded for co-op shenanigans. Make sure to rename it in explorer so you can tell it apart from the regular copy when browsing files, as well.

Adding onto this after further messing-around with a buddy of mine. This is probably known by now, but just in case it isn't, there's a couple of bugs we found:

First off, the trapdoor just before the boss door in Greenhorn Forest crashes the game when trying to load the jewel challenge area, every time. There's probably other ones too, but my friend and I jumped to just fighting the main world bosses after that happened.
Second, Player 2 cannot interact with breakable scenery or anything that requires doing a special move on an enemy to use, like the spin wheel to open the first gate in Greenhorn Ruins.
Lastly, the Black Jewel fight completely breaks if Player 2 hits the trapped sprites, since they attack P2 instead of the Jewel and then never respawn no matter what you do, forcing you to back out to the main menu

I hope the feedback helps, since this is a fantastic bit of wizardry you've managed, and I'm looking forward to seeing it develop further.

This [WARIO WORLD MULTIPLAYER MOD] was made possible by contributions to your [MR. BROCOLI] station from Viewers Like You. Thank You.

If there's anything I can do for you specifically, tell me.