Pure leaf mould is the finest garden compost. It takes up to two years to make the perfect batch but it makes the best and longest-lasting mulch, and can also be used as an ingredient in home-made potting compost. Leaves are rotted by a combination of fungal moulds and microbes that need air and moisture to do their work, so make the heap open at the sides and moist before covering with landscape fabric to weight it down and keep the heat in.
Put leaves through a shredder for faster results and, if you don’t have much space, black polythene bags work well if pierced to allow air in. If you’re very short of room for extra compost heaps, the leaves can be swept up and added to the general compost, but the results will not be fine enough for anything other than mulch.