You can use a Steel Greenhouse chipper or shredder. If you don't have access to one of those, you can spread them on the ground in a 2 to 3 inch layer and mow over them with your lawn mower.
Another way to break them up is to put them into a hard plastic bucket or trash can and go after them with your weed wacker. Spread shredded autumn leaves 2 to 3 inches thick.
Grass clippings are another free mulching material.
It's best not to apply freshly mown grass clippings to your garden because their decomposition process causes them to heat up enough that they may damage your plants.
Set them aside in a pile for a couple weeks before you add them to the surface of your garden soil.
Spread them no more than one inch thick. Grass clippings break down rapidly, so collect and save them each time you mow.
Hay can be a low-cost alternative if it is spoiled and therefore unsuitable for livestock.
Apply hay to a depth of 6 to 8 inches or 2 to 3 inches if you chop or shred it first. The downside of hay is that it usually contains weed seeds.
However, so-called second- or third-growth hay that has not gone to seed will eliminate that problem.
This is usually available after midsummer. An alternative is straw, which is weed-free but more Farm Tools expensive.