The microorganisms that break down organic material in your soil will happily do the same job in a pile of fallen leaves and plant trimmings. Your role as a composter is to provide the conditions for nature's crew of decomposers — the bacteria, fungi, and bigger creatures such as sow bugs and worms — to go to work.
Compost has four main ingredients: Browns, Greens, Air, and Water. Browns are dry, woody materials such as fallen leaves, pruned shrubbery, pine needles, newspaper, and so on. Greens are moist, nitrogen-rich materials such as fruit and vegetable trimmings, grass clippings, and fresh weeds. Air and water are the essential ingredients without which our industrious microfauna could not transform Browns and Greens into compost.
To make compost, simply combine Browns and Greens in more-or-less equal proportion, and make sure the pile has enough air and water. The formula looks like this:
Mix browns and greens.
Maintain air and water balance by keeping compost as moist as a wrung-out sponge.
Compost is ready to be used when it has a nice, earthy smell and a dark, crumbly appearance — like coffee grounds, only moister and not so uniform. If any items of food are still discernable, they can be screened out and added back to the bin.