This mod will replace Snake from Metal Gear Solid 1 costume with Quiet

1. Navigate to your "Steam\steamapps\common\MGS_TPP\master\" fodler

2. Extract this archive in there, so you should see two new subfolders: "Assets" and "QAR Tool"
  
3. Backup the file named chunk0.dat as it will be overriden

4. Open the "QAR Tool" folder in a new window (by the way, QAR Tool is from http://forum.xentax.com/viewtopic.php?f=10&t=12407&start=285)

5. Drag and Drop "chunk0.dat" over "MGSV_QAR_Tool.exe" to extract the contents from chunk0.dat.
Now you should have a new subfolder named "chunk0" as well as a new file "chunk0.inf"

6. Drag and drop the "Assets" folder from step 2 inside the new "chunk0" folder.  It should ask you to merge folders and override one file, namely "plparts_mgs1.fpk". Hit "yes" to everything

7. Last step: Drag and drop "chunk0.inf" (INF, not .dat) over "MGSV_QAR_Tool.exe" to repack the contents.  This will recreate a new chunk0.dat and should take around 1 minute