I just recently started working on some props for a Jinx cosplay and it was such a fiasco getting my hands on the files I knew existed, but couldn't seem to find anywhere So I decided to share so you don't have to spend hours trying to figure it out yourself and can get straight to making your cosplay (or modding if that's why you're here)
First, get your hands on a .skn to .obj converter.
See League of Legends uses their own format called .skn it's not compatible with any 3d modeling software that I've ever heard of so once you find your .skn file you'll need to run it through that in order to be able to use it.
You can find one here. If you're reading this months/years from now and the link is broken, I'm not going to fix it, I wash my hands of this business. Use your old buddy google, it shouldn't be too hard to find,
try here.
Second, get Dargon Manager
Dargon is a mod manager for League of Legends. It also doubles as a way to look into and dump the files from LoL.
You can download it off their website. Once you have it installed by following the directions on their website load it up and (with League already installed of course) you now need to find your desired .skn file. The folders seem to be organized by patch so if you're looking for something added recently (like Jinx!) the file should be somewhere at the bottom of the list. If you're looking for an old file, it might take some digging and I'm sure your old pal google and the modding community of LoL can help you find the exact folder that your .skn is hidden in. be careful though
there's more than one .skn per champ I've been told. As changes are made to the champ the .skn changes too so you'll want to find the most recent version of the .skn. When you've found the file you want, right click and dump the file into the same folder you dropped the converter from last step. Then all you have to do is click and drag it onto the .skn to .obj converter and it converts it for you in no time! Then you can import that bad boy directly into Pepakura or if you're like me and want to cut some unneeded bits off first, you can import it into 3DSMaxx or your desired 3D modeling program.
