Id like to create exhaust stain on my JU88, how do I create a wash and how do I apply it?, Bush or AIR brush?

If you go the AB route - use Tamiya's Smoke paint. It's a very translucent blackish washish color and I've used it for that before.
Another option is a dilute black wash. Use as wide a brush as you can and apply multiple layers. My washes are usually Tamiya flat black and water, maybe a touch of dish washing detergent to allow flow. I'd make a very dilute mixture and apply many coats till you're happy.

For the exhaust, you might be able to do it with pastels !

I was also going to suggest pastels but you could also airbrush it. If airbrushing it, I would use a 90-10 thinner to paint mix which lets you really vary the stain with respect to contrast and thickness. You could also use several layers of white, black and brown to build it up. Hope this helps you out.

Is your Ju88 a day bomber or night fighter? If the underside in black/dark gray you will want a stain with dirty white/light grey stain and a red brown stain in center. Pastels is how I would go. If you don't like it, just wash it off You can then seal the pastels if you put on sealing coat for your decals.

Agreed with the pastels unless you're really, I mean REALLY, skilled withthe airbrush. You're going to be doing thos to a virtually completed, painted and decalled model. Slip up and you have to go back over major areas to make he corrections. As mentioned above, overdo or mess up the pastels and you just wash it off. The also feather very nicely and maintain translucency until you really pile it on.

If i go with pastesl, do I use darkgrey and rust brown mixture?... Thanks for all the responses. It really helps