1) Download Open IV (http://openiv.com/), install it, set it up, whatever.

2) Go to "Grand Theft Auto IV\pc\models\cdimages" (Either in Program Files/Rockstar Games or Steam/Steamapps/Common)

3) Make a backup copy of "playerped.rpf" in case you decide to change your mind and remove the mod!

4) Using Open IV, go into "models/cdimages" and open playerped.rpf.

5) Click on "Edit mode", and then the green plus sign.

6) Navigate to this folder and add the "sus2_001_u.wdr" and "sus2_diff_001_a_uni.wtd" files.

7) Click "Edit mode" again and close Open IV. Congratulations, you've installed some shit!