First of all you want to download TDM cars or LW cars, for me I will be choosing TDM cars.
https://facepunch.com/showthread.php?t=1184807https://facepunch.com/showthread.php?t=1184807
You will need tortoise SVN.

Second of all downoad VTF Edit for later.
http://nemesis.thewavelength.net/index.php?c=149

Then you will need to port your TDM Car .PNG wire frame into PS or Paint / Paint.net, after this overlap the wireframe so you can see what you're working on.

Afterwards save this and then unlap the wire frame from the skin.

Open up GMOD Directory > Navigate to your addons folder and create the folder name.
For me its like this:
Ford_Transit
[inside of that make these below]
materials
models
tdmcars
ford_transit

inside the end ford_transit put your VTF and name it SKIN 2-15
Upload your image into VTF and select make a VTF makes a VMT
in your VMT file then you want to select it to look like this.

"VertexlitGeneric"
{
"$basetexture" "models/tdmcars/ford_transit/skin2" "$bumpmap" "models/tdmcars/shared/skin_nm"
"$halflambert" 1
"$phong" "1"
"$phongboost" "10"
"$phongfresnelranges" "[0.5 0.4 5]"
"$phongexponent" "200"
"$rimmask" "1"
"$envmap" "env_cubemap"
"$envmaptint" "[0.1 0.1 0.1]"
!hdr?$envmaptint "[ 0.2 0.2 0.2 ]"
hdr?$envmaptint "[ 0.3 0.3 0.3 ]"
"$normalmapalphaenvmapmask" 1
"$envmapFresnel" "1"
"$blendtintbybasealpha" "1" "$envmapcontrast" ".5"
"$surfaceprop" "metal"

"$rimlight" "1"
"$rimlightexponent" "25"
"$rimlightboost" "0.5"
"$phongalbedotint" "1"
}

VMT AND VTF Folders should be called skin2 or whatever yours will be.