I want to make an GTAVC Squalo, which appeared in mission "The Fastest Boat" (which is pretty hard to me..)
Okay, what do we need.
#GTA (from III to SA, depending on the vehicle), I'll be using GTAVC, obviously.
DFF Viewer program, can be found here
IMG Tool , here
It's all in russian, but don't fear, just scroll down and find "???????: imgtool2.zip"
Click on imgtool2.zip, then wait until you see a green button. Then press it.
SimplePlanes
Patiency
This tutorial (pretty unnecessary, yea?)#
Now, open the IMG Tool. Then press File - Open and then go to your GTA folder.
Now, go into "models" folder and open gta3.img. Press F2 and type your ingame vehicle name. For Squalo, it is "squalo.dff". Select it, right-clock on it and select Extract. Select a path, you know what to do. All right, now you can close the IMG Tool and open DFF Viewer, in right-down corner click Open, then go to your path and open your dff file.
Volia! You have now exported an model and put it into a viewer. Now you can use the model to pinpoint your build, just use the model as a reference you can rotate, explore and use.

Method 2: Download ZModeler, do the actions with IMG Tool you seen before, but now open ZModeler. If it crashes with an error, just install all vcredist things in the second folder, from 1 to 5.
After opening, press File - Import, select DFF as a file type and open your exported model. You can see here all the variety of supported file types, starting with obj and ending with 18 Wheels of Steel and GTAIV, NFS and BF2. Now you can orbit with LMB and pan with MMB.
Good luck!