If your doing a water change every 2 weeks or something. Then clean the filter then, when you do the water change.
Once you have taken your old tank water out, just get the filter sponges and give them a little squeeze in the old tank water, then pop them back in the filter.
Don't use normal tap water as you can kill the beneficial bacteria that builds up on your sponges in your filter.
when you notice a reduction in flow its time for a clean.
did 1 of mine last night that hadnt been done in about 4 months. it all depends on the stocking etc, lightly stocked tanks wont need the filter cleaning as much as over stocked tanks.
yes sponges first, ceramics etc in the middle and ceramics and floss in the top basket.