Mulch is measured in cubic feet. As an example, if you have an area 10 feet by 10 feet and you wish to apply 3 inches of mulch, you would need 25 cubic feet. Determine what mulch material to use and purchase or accumulate what you need. Mulch can often be purchased bagged or bulk from garden centers. Bulk may be cheaper if you need large volumes and have a way to haul it.
Bagged mulch is often easier to handle, especially for smaller projects.
Chop with a lawnmower or shredder. Whole leaves tend to compact if wet or blow away if dry. Chopping will reduce the volume and facilitate composting. Compost leaves over winter. Some studies have indicated that freshly chopped leaves may inhibit the growth of certain crops.