This is the legendary Forumsmod. It's about enhancing the Multiplayer experience, while not going to far away from the known base game and not to overburden it. If you're seasoned with vanilla, you should have no problem to quickly adapt to this mod. There are a lot of quality of life changes and small improvements, but there are big ones as well like a completely overhauled china and india. Every nation got more decisions and got re-balanced, so it should be more about skill and not starting conditions, whether you succeed or not. I've tried to be as historical and add as much flavor as possible, while staying competitive and with an improved game performance. But the latter is more important, so sometimes the game is even more unhistorical as vanilla, because the game mechanics don't allow it to make it like described above.
Draw the mod from "Downloads" to "C:\Program Files (x86)\Steam\steamapps\common\Victoria 2\mod". There you unpack the file, preferably with the WinRar program (if you don't have an unpacking program, then download it for free). Then you click on play in Steam and on the mod in the bottom left of the window, if there is a check mark it should work.