Ok so i am going to assume you are talking about an Electrolux filter
that is basically a rectangle shape with a hard plastic outer and folded corrugated filter material in the middle. The link I put to Electrolux filter would be a good example. Basically I would caution you against washing it. I know the packaging says that it is washable but what I have found is that the pores of the filter material are too fine for water to pass through. What will happen is that the vacuums suction will wedge tiny bits of dirt and dust in the pores and water will not necessarily flush it out. What is far more likely to happen is that it will wash out a fair amount but leave a portion in the filter to set and therefore continue to block suction.
You are better to use an air compressor or simply tap the filter out on a piece of newspaper and change it every year or so.
Of course if your filter is more like a Electrolux UltraActive filter
which is basically foam then you simply rinse it out like a kitchen sponge and make sure it is bone dry before putting back in the machine.