Interesting fact: Leaves are basically carbon
Top tips
Pile up your leaves in an unobtrusive corner of the garden (or you can put them in bags - but make sure you put aeration holes in it)
Speed up the leaves' rate of decomposition by running a lawn mower over the pile a few times.
Make sure they are kept damp
Turn your pile every few weeks with a garden fork or spade to hurry up decomposition