If you have the room to store them in the open - say, behind a garden shed - the simplest thing to do is to tie the bag tops and make a few holes in the sides of the bags to allow oxygen in and leave them to rot. The tops need to be closed to prevent an excess of moisture as this wont help; although the leaves need to be damp. The oxygen is necessary to the composting process.
You could transfer the leaves to a compost bin if you have one, but the cost of buying for this only is a bit pointless.
It will take a few months but the volume will reduce and you should end up with decent compost.