I've got a cleanable panel air filter (cant remember what make, but it looks like the green HKS one) on my car, but not sure how to do it. Use a vacuum, stick it in the washing machine???

I got the large bits out of it, but it is quite black.

 

K&N do an airfilter cleaning kit with good instructions. Dont just wash it as you could push any dirt further into the filter.....