how do you create...

Representing fading without an airbrush will be very hard to do. You could look at getting a dark and light shade of the same color of spray paint. Spray on the dark color and then try to target areas that would get more sun with the lighter color and lay on a thin coat in those targeted spots. But that will be hard to do with the kind of wide spray pattern you'll get from a can. If you are interested enough in these kinds of techniques to be considering them, I encourage you to invest in an inexpensive airbrush and a couple cans of air. It doesn't have to cost you much more than the cost of a few cans of spray paint and you could get the kind of results you are imagining, which will end up encouraging you in your hobby rather than discouraging you.

Without an airbrush, dirt on windshield is hard.  But for dust on rest of car, drybrushing is not bad, but takes some skill.  These days you can get an airbrush and compressor for the price of three or four kits, and is money well spent.  Or, if you have some source of compressed air, an air tank lowers price even further, though it can be a bother filling the tank frequently.

For a light dusting effect on the windshield, tape up the half circular area that the blade will cover and lightly spray clear flat over the windshield. Let it dry then remove the tape exposing the clean wiper area. The rest of the windshield will be semi opaque looking like light dust.  

For the windshield you can use a thin buff wash or you can apply pigments to the windshield like real dust.