I had to first get the model somehow then import it into 3DS Max then use GIMS Evo V to convert it into a GTA V Asset. Then i used GTA 5 Map Helper to make the files needed for it to appear in-game. I then configured the files to make sure they worked. I had some help from a bunch of modders that told me what i needed to do to get it in the game.
Edit: If your planning to do this yourself then your only option is to use 3DS Max or ZModeler 3, both paid programs :(
You can however get 3DS Max for free if you are a student. Go search it up on google.