I already helped many of people who try to made these. One more wouldn't hurt but I must warn you it's really difficult. Before I start. I must tell you that I do most math in papers. And everyone I teach run away at first sign of complication. So brace yourself.
Here we go. Everyone have problem with repack because there is no repack application(there is, Redduck have one) So then in order to make this modification happen you had know how the file work.
below is how my method work.
Encrypted package > decrypt > Encrypted file > decrypt > compressed image > decompress > image(editable at this point)
then in order to make it work inside the package again you need to
compress the image > encrypt > get all files that were in package together and encrypt the whole package again.
First, In each package(upk) contain encrypted files. Textures or skins included. This is the first problem. You need to learn how to decryption the file.
then how to learn or know about decryption and encryption ?
I suggest you read RSA algorithm to learn the basic of it.(at this point you should have knowledge of Computer Sercurity : Encryption) But any of encryption algorithm always have the key(in this case : an equation). This key obtain by compare between the files.
For example : you need an old one and updated one, SA58 Para, W098.upk is the most frequency update at a time). To obtain the key is required the mathematics called differential equation in order to solve and obtain key.(the key is an equation that can be use for decryption and with a little work it can also encrypt the file as well)
To make short, compare, use differential equation to find the key . Easy does it ?(not really)
second step is the decompress images. This is easy. There are a lot of application that will help you make file.texture2d into any image extension. But I suggest you make yourself one. Because the whole encryption and decryption required a bit of coding to help. Unreal use zlib image compression algorithm.
Go look for one or make. Don't be lazy and use umodel because it won't bother compress the texture2d back to former state.
third part : PARTY Time.
Paint, Photoshop whatever you good at but note this. It's difficult to encrypt it back when you modify it a lot because of size and pixel mapping. You can see I made only 1 unique skin here, the Silver M4A1, because it's extremely difficult to made one when the whole image file is changed It's took me a very long time to learn from Skin maker and made one that why the rest of released skin are not unique.
fourth step. Compress it back.
fifth step. Encrypt it.
last step gather the whole file in package and make them whole and encrypt tehm then test it in game.
It's quite frustrate when the file didn't work. It happened to me a lot, more than I care to count so be patient with these trial and error methods. Here is what skin maker told me. "I don't care if you have the Skills or knowledge. Without will and passion you won't gonna success."(actually it's require a lot of knowledge) so I hope you have the will and passion to succeed. If it's easy then It'd happened long time before SEA AVA.(the server opened almost two year and the skin maker made first change of texture there, he also post it here on MPGH, the Asia rank for ijji server.)