Alright, here we go...
You can see the acrylic craft paint. I mix a batch in an old paint jar. This will last for a bunch of builds!
Brush on some wash...
Rub it in...(you will need to brush some, next to high spots, that your finger just can't get into)
As it starts drying, wipe in the direction of airflow (sometimes, some streaks will remain after wiping away the excess...further enhancing the weathering)
Once dry...just takes a few minutes. Wipe away excess with damp cloth...
Before/after
With small models, such as a 1/72 P-51...you can wash the whole thing at once. For bigger stuff, I work in sections (as with this example)
Once wiped away, sometimes you'll get some inconsistancies. I use a smaller brush to add a little more wash and let it dry, wipe away...
left side washed...
Hope that helps ya some!