Remove the foam, Wash it out with Warm soapy water.. I use a bit of dishwasher liquid(really cuts the grease). rinse it well... sqeeze all the water out... I roll it really tight in a paper towel.. Let it air dry,,, then re-oil it and intsall .
However... New filter foams are really cheap, and lots of the "Big" guys here say that you will never get all the fine dirt out anf it can later get sucked into your engine.. If your running a reall expensive engine, I wouldn't take any chances.