This is almost too easy. If leaves are left in a pile and exposed to moisture, they will compost into a dark crumbly mass that is perfect for improving soil texture and moisture retention. Leaf mould is an effective conditioning material for mulching the lawn, and a nutritious soil amendment for planting holes. (Tree leaves contain about twice the mineral nutrients found in composted animal manures.) Mature trees pull minerals from deep soil lying just above the rock underbase. Most of these rock minerals are used in the leaves, in the process of photosynthesis. When tree leaves degrade, they return minerals to the soil. For example, sugar maple leaves are about 5 per cent mineral content, while pine needles are about 2.5 per cent mineral. Both contain calcium, magnesium, nitrogen, phosphorus and trace elements. Using a mix of different leaves results in a broader range of minerals in the finished leaf mulch.
The simplest method for producing leaf mould is to rake fallen leaves into a loose, low pile, somewhere out of the main flow of wind. I gather drifts of leaves under the low branches of large conifer trees, and let them sit there to compost until needed. For a pile out in the open, a long piece of chicken wire laid over the top and anchored with bricks or rocks on the corners would prevent wind from scattering the leaves.
The leaves on the bottom of the pile will be the first to degrade and produce crumbly material. In summer, be sure to frequently spray water from the hose over the piles to replenish evaporated moisture (fungi and bacteria need a moist environment to work effectively). I usually leave the pile alone for a year, and then fork it into a wheelbarrow for use in the garden. Adding a nitrogen source to the pile of leaves speeds up the process. A few sprinklings of lawn fertilizer mixed into the leaves is sufficient. Bloodmeal also works, but it will attract animals.
It’s also possible to produce leaf mould in large plastic garbage bags. Be sure the leaves are moderately moist before putting them into bags, along with a sprinkling of lawn fertilizer. When the bags are filled (and don’t pack them too tightly, the leaves should be able to move around inside), leave the tops open and poke holes in the sides to allow air passage. Store the bags outdoors, perhaps along a fence or behind a garage and out of sight. Check them in early summer; you’ll probably find partly decomposed leaf mould. The material is certainly usable at any point in the process; it’s not necessary to wait for the leaves to entirely break down.
And did I mention that this most perfect soil amendment is entirely free?