There is a person that I despise holding back this information and charging money for it. I said screw that, figured it out myself, and I'm now releasing this for you all to enjoy.
This will be quite long, as I'm going to be describing each process here.
What you need:
The Game
HxD (google it, it's free)
Patience
A brain
The General Process
Open up the F13 save file with HXD, it's named SCCharacterSelections.sav and is located in
C:\Users\(yournamehere)\AppData\Local\SummerCamp\Saved\SaveGames
This is where your game saves every selection, aka Jason skins, animations, emotes, as well as counselor clothing and perks. There are so many different things you can do here, including triple stacking perks, but I won't be covering that unless requested enough.
(if you look at this tutorial for everything else, it should be pretty easy to figure out.)
Now, the tutorial for EVERYTHING.
First thing we'll do is simply replace Part 2's skin with the Retro Jason skin. In order for both of these skins to show up, you NEED to customize both part 2 and 3 for them to show up. In this case, I customized Part 3 with Retro, and Part 2 with his bloody skin.
In this process, you can scroll through, or you can search by hitting CTRL + F. First, we'll copy the skin of Retro. Find it by... well, searching "Retro" with the command I just told you.
It should be the only instance in the whole file if yours is unmodified. You'll see the line
F.../Game/Characters/Killers/Jason/J3/Skins/J3_Skin_Retro.J3_Skin_Retro_C
Copy this part.
PRO TIP: That "F..." at the beginning of this line, as well as every other one, is very important. If you don't copy it, the game WILL crash.
Now, find the Jason you want to replace the skin of. If you selected a skin for him, you'll obviously find it under the "SelectedSkin" text in his section.
For Part 2, the heavily bloody skin is named
R.../Game/Characters/Killers/Jason/J2/Skins/J2_Skin_Blood_Heavy.J2_Skin_Blood_Heavy_C
Take that copied text of Retro Jason's skin that you just got, and replace that line of Part 2's skin.
You've just done it, save it and load up the game, there is Retro Part 2, in all his glory.
Now, there are a lot of things you can do this SAME EXACT PROCESS with.
Counselor clothing? Check. Search the one you want to replace a texture with, find the one you want to replace, do it. Done.
Kill animations? Want Machete kills on Part 3? Now you can, copy Part 7's lines over to Part 3. Boom.
Remember, things will NOT show up unless already customized in the game. If you don't have kills selected for part 7, obviously the slots there will just say "None." It's far easier to work with when everything is customized already, and ready to be modified.
I cannot stress enough HOW IMPORTANT the 3 dots and the character/symbol that come before the file name are. The game WILL crash if you do not copy this part over on anything you are replacing or changing.
Well, that's it! If you have any questions about changing certain things, just ask, I'll be around to answer.