BritCop

depends on what you want to make. Obviously you don't want to reinvent the wheel. everybody has made the cars already.
 
Find a specific vehicle you want to try first.  Then make a skin/Livery for it.
 
You will need the template file for the vehicle. A photo editing program (paint or Photoshop) i use Photoshop. Open the template into Photoshop and up the image size to 4096 (currently at 2048). Figure out how you want it to look. Using Photoshop and the pen tool, create a new layer and select the area you want to change color. then add color. create new layer and add text. keep repeating until you get what you are looking for. It's a long process. (Adjust the layer transparency so you can see the template underneath) Once you get it looking they way you want it, Save it as a PSD (Photoshop format). Now save/export it as a png and name it (if you are using the charger template) charger_sign_1.png (This is for the image size 4096) Now go back and resize image to 2048. Save/export this and name it chrg_sign_1.png (this is for the image size 2048)
Now, Open OPENIV (download from lspdfr.com downlaod page) and you will insert these images into the 2015 charger ytd files (ytd and ytd_hi).
To learn how to add vehicles to GTA5, follow Albo1125 tutorials on youtube and he walks you thru it all.
 
This is obviously a quick breakdown of what you are doing and by know means covers everything. I am currently doing this for Gump's Charger. I will create a video tomorrow with some of the steps.
This is what I have got done so far. But have already made drastic color changes and such in the last 2 hours.
https://www.youtube.com/watch?v=ZJzYC0XlrF0
 
I am currently using GUMP's 2015 Dodge Charger