So, in the beginnings, you need a program. The programs name is: gca_e. With this program, you can extraxt the .dat files.
1. - How to extract the .dat files???
So, we are now extract the xfile.dat. The first, you need to rename xfile.dat onto xfile.gca. Then you can extract the xfile.gca like this: File/Extract file. When the extract finished, you can search for files in the xfile folder.
2. - How to Skinning???
The serious lesson. We need a converter program, to convert TGA files onto BMP, or anything much. Then, you can create your own skin. But you need to know, whitch picture witch part if the model. But that's always other.
3. - How to setup your skins???
This is the last and easyest lesson. You just need to compress folder like this: File/Compress folder. Than you need to find that folder where you skin is (Caution!: You need a backup file from the first .dat files, and you need the extracted .dat files, to copy to the right place. So: when you update the em31 folder, you need to copy the em31 folder into the xfile/em folder!!!). Then, when the compressing is finish, you need to rename your new .gca file to .dat file, then copy into your RE4 folder. Thats all about skinning.