Introduction
Hello, I am r3pwn and today, I guess, I will begin to teach you mobile source apps modification. Don't worry, it only consists of a few steps really.

Picking a file
First you start with picking a file you want to modify. Then, extract it from your app's obb file.

What to do?
Well, now, you make your changes to the file and put it under "/data/data/com.nvidia.valvesoftware.portal/files" or "/data/data/com.nvidia.valvesoftware.halflife2/files", depending on which game you own. You can find my modified files for Portal here and my modified files for Half-Life 2 here. For my Portal files, extract the zip to the "portal" folder in Portal's app files. For my HL2 files, extract the zip to the "hl2" folder in HL2's app files.

What do we do next?
Now it's time for the final step. Permissions changing. Hook up your SHIELD via adb or open a Terminal Emulator and issue the following commands (Please note that "yourgamefiles" is /data/data/com.nvidia.valvesoftware.portal/files is you applied the mod to Portal or /data/data/com.nvidia.valvesoftware.halflife2/files if you applied the mod to Half-Life 2):

su
cd yourgamefiles
ls -na

After that, you need to look for the 5 digit number that should appear twice on most items. Let's call that our "5dn" (stands for 5 digit number). Then issue the following commands:

chown 5dn *
chown 5dn */*
chown 5dn */*/*
chown 5dn */*/*/*
chown 5dn */*/*/*/*
chown 5dn */*/*/*/*/*
chgrp 5dn *
chgrp 5dn */*
chgrp 5dn */*/*
chgrp 5dn */*/*/*
chgrp 5dn */*/*/*/*
chgrp 5dn */*/*/*/*/*

(It's okay if one or more of those commands gives an error of "No such file or directory".)
After that, your permissions should be fixed. Start up the game and see if anything looks/functions any different!

Happy Modding!
- r3pwn

P.S. If this guide was too complicated for you OR you didn't understand it OR it didn't work for you, don't worry. I am making an automated patcher application as we speak. :)