1) Extract the materials folder in MARPAT Desert to your csgo folder.  AFTER extracting the MARPAT Desert files, If you want to use the NOMEX Tan variant, repeat step 1 for the NOMEX Tan materials.

IMPORTANT NOTE:
ALWAYS extract the MARPAT Desert files BEFORE using the NOMEX Tan variant.

2) In your csgo folder, make a back-up your pak01_dir.vpk file.

3) Open the pak01_dir.vpk file using Notepad++ (do NOT use the default Windows Notepad).

4) OPTIONAL: for quick installation, copy the pak01_dir.vpk file provided in the archive to your csgo folder.

IMPORTANT NOTE:
Using the step 4 shortcut WILL DISABLE ANY OTHER SKINS YOU ARE USING.  Skip step 4 and manually edit the pak01_dir.vpk file using the following instructions to retain the simultaneous use of other skins.

5) Search for the listed lines below (and ONLY the listed lines below) and change them as instructed.  The lines are NOT case sensitive, so don't worry about upper or lower case.

$baseTexture "models/player/ct_st6/st6_face"
Change to:
$baseTexture "models/player/ct_st6/mfr_face"

$bumpMap "models/player/ct_st6/st6_face_normal"
Change to:
$bumpMap "models/player/ct_st6/mfr_face_normal"

$phongexponenttexture "models\player\ct_st6\st6_face_exponent"
Change to:
$phongexponenttexture "models\player\ct_st6\mfr_face_exponent"

$baseTexture "models/player/ct_st6/st6_lowerbody"
Change to:
$baseTexture "models/player/ct_st6/mfr_lowerbody"

$bumpMap "models/player/ct_st6/st6_lowerbody_normal"
Change to:
$bumpMap "models/player/ct_st6/mfr_lowerbody_normal"

$phongexponenttexture "\models\player\ct_st6\st6_lowerbody_exponent"
Change to:
$phongexponenttexture "\models\player\ct_st6\mfr_lowerbody_exponent"

$baseTexture "models/player/ct_st6/st6_upperbody"
Change to:
$baseTexture "models/player/ct_st6/mfr_upperbody"

$bumpMap "models/player/ct_st6/st6_upperbody_normal"
Change to:
$bumpMap "models/player/ct_st6/mfr_upperbody_normal"

$phongexponenttexture "\models\player\ct_st6\st6_upperbody_exponent"
Change to:
$phongexponenttexture "\models\player\ct_st6\mfr_upperbody_exponent"