I remember a big, dead, fallen White Ash tree in my families' woods when I was a kid. I would climb onto the prostrate trunk and walk along it, 3 or 4 feet off the ground- it was that big. Now, 50 or so years later, it is just a slight, short rise in the ground, having completely turned back into soil by Mother Nature. It's covered with moss and ferns, with young trees and shrubs growing out of it.
Brush piles that I made years ago from branches of hardwood trees harvested for firewood, and softwood trees for lumber have long since disappeared.
That's composting on a long, slow timeframe.
I've read a lot of articles in the past on how one needs to create compost piles using a strict 70 to 30 or some other ratio of dry, brown, carbonaceous ingredients to green, wet, nitrogenous materials. That's all well and good, but it doesn't need to be so regimented, and I think it might have steered people away from doing it, making it sound so technical.
Put whatever you have on hand and mix it up. Make it as big as you want. Turn it every few weeks. Cover it with a tarp when it rains too much (THIS SUMMER). Water it if we're in a drought (THIS SPRING).
Stick a dung fork in it, wait a few minutes and take it out. Grab the tines- they're probably hot.
Continuously add to it as material becomes available.
Grass clippings, hay, straw, weeds (best before they go to seed), potato peels, wood chips, egg shells, tea bags, left overs, chicken bones, apple cores, small branches and sticks, barnyard manures, wood ashes, dead chipmunks, some lime, old compost, soil, and everything from the vegetable and flower gardens at the end of the growing season.
Autumn leaves are a great addition, but they need to be mixed in thoroughly with other ingredients, so that they don't mat down and exclude oxygen. I pile my leaves separately and add them to my compost pile gradually after 6 months or so, when they've broken down a bit already.
Billions off aerobic and anaerobic bacteria, fungi, yeasts, millipedes, sow bugs, beetles and earthworms- these and countless others are all working (and adding their waste and bodies) to make compost.
Let it happen.
It's all I fertilize my gardens with- nothing else.