When you have decided and built the model of your choice give it a finish coat of enamel paint it must be enamel paint otherwise this method will not work, after you have sprayed your top coat  you need to get some white acrylic paint a dishwasher tablet/powder i prefer the tablet the reason being when it is crushed up it's granuals are of a bigger size not random like the powder is, let the enamel fully cure when you are satisfied it is dry get your white acrylic paint ready in the air brush and spray the model in the areas that you want the white wash to be dont spray heavy coats just enough to cover the base coat leave it to dry for 15 minutes only no more. while you are waiting then get one of those squeezy bottles with a trigger fill it with clean water, crush up the dish washer tablet as fine as you like but leave some bits bigger?

so you are now ready to do the method after the 15 mins have passed spray the whole model with a fine misty coat of water not dripping wet, then apply the d/w granuals to the white painted areas i just sprinkle it on  putting more of it on walking/climbing areas that would see a lot of foot traffic leave the powder on for 30 mins for it to attack the acrylic paint you can see it doing this, when you are satisfied pick up the model carefully not touching the areas you applied the powder to, go to the kitchen sink turn on the cold tap so you have a fine thin stream of water no pressue put the model under the stream of water and let it wash of the powder residue. then put the model on a paper towell to dry thourghly and there you have it a random looking white wash finish, then when it's fully dry give it a coat of future then do your decals washes/weathering etc.