A properly built compost heap smells of almost nothing — a faint, pleasant woodland scent of leaf mould at most. If yours is whiffing of rotten eggs, ammonia or the inside of a wheelie bin on a hot day, don't panic. That smell is information, and it's usually telling you one of two things: the heap is too wet, or it hasn't enough air in it. Fix those, and the smell disappears on its own.

Home composting is one of the most satisfying parts of growing your own, and it's almost entirely free. Here's how to get it right from the first forkful.

Greens and browns: the balance that does the work

Think of your heap as a fire that burns slowly. The "greens" are the fuel — soft, wet, nitrogen-rich stuff that rots quickly. The "browns" are the structure — dry, carbon-rich material that lets air in and soaks up moisture. Too much green and it turns to slime; too much brown and nothing much happens for a year.

• Greens: grass clippings, vegetable peelings, fruit scraps, tea leaves, coffee grounds, spent bedding plants, cut nettles, comfrey, young weeds before they set seed.
• Browns: torn cardboard, newspaper, egg boxes, toilet roll tubes, straw, autumn leaves, woodchip, sawdust, chopped prunings, shredded paper, wool offcuts.

Aim for roughly two to three parts browns to one part greens by volume. That's more brown than most people use at first. If in doubt, add cardboard. Kitchen peelings are wet and dense — every bucket of them wants a couple of good handfuls of torn cardboard or a few sheets of scrunched newspaper alongside.

Building the heap: size, site and the first fill

Size matters more than people expect. A heap of about a cubic metre — a metre wide, a metre tall, a metre deep — holds heat well and is still turnable. Smaller heaps work, but slowly, especially in a cold winter. A plastic dalek bin is fine for a small garden, provided you empty it out and rebuild the contents every few months rather than just tipping things in forever.

Sit the heap directly on bare soil if you can, not on paving. That way worms, beetles and beneficial microbes can move up into it from below. Fork the ground over first to loosen it, and lay a 15cm base of coarse browns — woody prunings, straw, scrunched cardboard — to keep the bottom aerated.

Then build in layers: a 15cm layer of greens, a 15cm layer of browns, a light sprinkle of soil or old compost, and repeat. Mixing as you go is even better than layering if you've the energy. Keep kitchen waste buried in the middle of the heap rather than dumped on top, which is the single best way to keep fruit flies away.

Turning: how often, and how hard

Air is the whole game. Turn your heap every two to four weeks through spring and summer, and perhaps once or twice over winter when everything slows down. Fork the contents from one bay into another, making sure the outside edges end up in the centre where the heat is. That's where the composting actually happens fastest.

If you can't turn — a dalek bin or a small plastic cone — use a compost aerator or simply push a broom handle deep into the heap in several places and wiggle it about. It's not as good as a proper turn, but it makes a real difference.

You'll know it's working when the middle feels hot to the hand and steams on a cold morning. That heat is the sign of a healthy heap, not a fault.

Common mistakes and how to fix them

• Slimy, sour and smelly: too many greens, not enough air. Add browns, fork it over, and leave the lid off for a few days in dry weather.
• Bone dry, nothing happening: add water — a watering can over each layer — plus a few handfuls of greens or grass clippings.
• Fruit flies: bury kitchen scraps in the middle, wrap them in newspaper first, and avoid adding cooked food or meat.
• Rats or mice: never add meat, fish, dairy or cooked food. Use a sealed bin with a base plate, or line the bottom with fine mesh.
• Thick layers of grass clippings: the classic mistake. They mat down into an airless slab. Mix them with cardboard, leaves or woodchip, or spread them thinly as a mulch instead.

When it's ready, and how to use it

Depending on the season and how often you turn, your compost will be ready in anywhere from three months to a year. It's done when it's dark, crumbly and smells earthy, and you can barely recognise what went in. A few eggshells and woody twigs will always survive — sieve those out and throw them back into the next heap.

Use it as a mulch around fruit bushes and perennials, fork it into vegetable beds, or mix it roughly half and half with garden soil and a little grit for containers. It won't feed your plants as strongly as a bought fertiliser, but it builds the soil structure that makes everything else work — water-holding, free-draining, alive with worms. Sieve a finer batch for seed sowing and potting on, and you'll wonder why you ever paid for the stuff in bags.