It all depends on how much work and money you want to put into it.
The easy way is to park it next to a sand pile when its windy. Using a sandblaster at a distance can work too. Or you can wet sand.
I suggest wet sanding and using red/maroon scuff pads. The idea is to remove the original paint in the areas where it would naturally wear off, Ex) the door window ledge from years of the driver placing his arm out window, body lines, roof etc. Its a good idea to look at trucks of the same vintage and see how they show wear and recreate that.
Start with a 400 Grit and work your way up to a 1000 grit. The higher the grit the smoother it'll be and less noticible that it was faked.Make sure you use broad strokes and a soft pad behind the paper so you don't get finger grooves.
To create surface rust, sand all the way to the metal(again following natural wear patterns) and put vinegar on it. Once you get enough rust wash and wipe with Wd-40.