You need a copy of the original Pokémon Emerald. No downloads here as it is kind of not perfectly legal to distribute those games without Nintendo's permission. And Nintendo does not tend to give out permissions for distribution channels beyond their control. So, you need to find the ROM image somewhere.
If you have the original cartridge, there are some ways to copy the game off it (easiest with a Nintendo DS / DS Lite and a programmable game card, a bit more difficult if you have to build a copying device), but if you do not know how that works already, chances are you will not be able to pull it off anyway. That is the only way that is, under some legislations at least, unquestionably legal.
The more common, though legally questionable, way is to download this somewhere on the internet. I cannot tell you where exactly, but search engines should be able to if you ask them for a Pokémon Emerald ROM. You are technically not wronging anyone if you own the cartridge, but those sites do not discriminate between those who own the game and those who do not, so game companies and the legislators they buy tend to oppose those sites. Downloading copies of software you already have a license for is often a legal grey area, under some legislations it might be more on the black side. Even then, however, law enforcement only really cares about those who offer the download to those without the original game.
Now that you have a ROM image of the original game, you need to patch it. The author of the nudist version cannot just share the game either - it still contains lots of stuff from Nintendo's version, and unless Nintendo says OK, you cannot just share this on the internet. So what the author did is a clever little trick. He compared his version to Nintendo's version and instead of uploading the full version, he only uploaded the parts that he changed himself. Nintendo does not own that stuff, so he can share that without asking. That means you now need to put the game and the changes back together to get the mod. This is where IPS patchers come in. You need to get a program - that makes Chromebooks a bit problematic, since you cannot easily run traditional programs on them. If you have an Android phone, you can run the patcher on that. You open both the original game and the patch in there, and it spits out the modded game. Then just open it in your emulator and play away.