Four products, and twelve weeks before you judge them

Four products will do almost all of it. A cleanser, a moisturiser, a sunscreen, and one active ingredient at night. You can buy all four in a supermarket. The version that costs ninety pounds a jar and the version that costs six often share the same first five ingredients, in the same order, on the back of the box.

That ingredient list is the useful part, and almost nobody reads it. Ingredients are listed by weight, highest first. Water is nearly always number one. If the thing the front of the bottle is shouting about appears seventeenth, after the fragrance, it's there so it can be printed on the front.

Start by reading the back of what you already own

Go and get everything off the shelf. Line it up. You'll probably find eleven bottles, seven of them half used, three of them bought because your skin was angry and you wanted to do something about it that afternoon.

Sort them into two piles: things you've used consistently for more than a month, and things you haven't. The second pile is the answer to why the routine isn't working. Nothing in skincare works in a fortnight. Twelve weeks is the honest minimum before you decide whether a product did anything, because that's roughly how long it takes for changes in turnover and pigment to show up on the surface of your face.

Don't throw the second pile out yet. Just put it in a drawer. If you don't reach for any of it in three months, it goes.

The cleanser is the cheapest decision you'll make

Your skin should not feel tight after washing. If it squeaks, the cleanser is stripping you, and everything you put on afterwards is repairing damage you paid to cause.

A gentle cream or gel cleanser, lukewarm water, thirty seconds. Once a day is enough for most people, at night, to get the sunscreen and the day off. In the morning, water on a flannel is fine unless you're oily.

This is the product where spending more buys you the least. Cleanser is on your face for half a minute and then it's down the drain. Expensive actives in a cleanser are money in the plughole.

Moisturiser is a delivery system, not a miracle

The workhorses are glycerin, hyaluronic acid, dimethicone, squalane, ceramides, shea butter and petrolatum. Some hold water in the skin. Some sit on top and stop water leaving. A decent moisturiser has one of each kind and not much else.

Watch the jar sizes. A face cream comes in 50ml. An eye cream comes in 15ml and costs the same or more. Work out the price per millilitre and the eye cream is usually three or four times dearer for a formula that's often thinner. The skin around your eyes is genuinely more delicate and there are ingredients you wouldn't slap right up against your lash line. But you don't need a separate expensive tube for it. Use your moisturiser, gently, and keep the strong actives a finger's width away.

Collagen in a cream is marketing. The molecule is far too large to reach the layer where your own collagen lives. It works fine as a surface moisturiser. You're just paying for the word.

Sunscreen is the one place to spend, and the one people get wrong

Every bit of the anti-aging budget is wasted if you're not wearing sun protection. Not because of a scary statistic, but because sun damage is the thing the expensive serums are trying to undo.

The mistake isn't the SPF number. It's the amount. Most people apply somewhere between a quarter and a third of what the bottle was tested with, which turns SPF 50 into something far weaker. The rough guide is a quarter of a teaspoon for the face, or two lines of product along your index and middle fingers. It's more than you think. It looks like too much in your palm.

The SPF in your foundation doesn't count, because you'd need to trowel on a frankly comic amount of foundation to hit the tested dose.

So buy a sunscreen you actively like wearing. If a fifteen-pound one feels pleasant and a five-pound one feels like candle wax, buy the fifteen-pound one and use it properly. A cheap sunscreen used thinly is worse value than a dearer one used correctly. That's the one place I'd let comfort beat price.

Reapplication is where everyone, including me, gets lazy. If you're at a desk away from a window, once in the morning is realistic. If you're outside at lunchtime or driving west at five o'clock in June, it isn't.

Pick one active. One.

The shortlist:

• A retinoid for texture, fine lines and congestion. The best-evidenced thing on the shelf.
• Vitamin C, usually L-ascorbic acid, for brightness and pigment. It oxidises. If yours has gone orange, it's done.
• Niacinamide at around 5 percent, for redness and oil. Gentle, forgiving, cheap.
• Azelaic acid for rosacea-ish redness and stubborn spots.
• Salicylic or glycolic acid for blackheads and dullness, two or three times a week, not nightly.

Buy one. Use it for twelve weeks. The reason is boring and completely practical: when you start three at once and your face reacts, you have no idea which one did it, so you bin all three and start again with something else.

Retinoids need a slow start. Twice a week, a pea-sized amount for the whole face, on dry skin, at night. Build up. Expect some flaking around week two or three. If you're burning, peeling in sheets or stinging, you've gone too fast, and going back to plain moisturiser for a week isn't failure.

If you're pregnant or trying to be, skip retinoids and ask your GP or midwife what's suitable. Same conversation if you're on prescription acne medication.

What you can cheerfully ignore

Toners. Essences. Ten-step routines. Sheet masks, except as something pleasant on a Sunday. Jade rollers. Most of what's sold as a "treatment" that isn't one of the actives above.

And a word for the drawer of samples. Sachets are a fortnight of nothing. They're too small to tell you anything and they exist to make you buy the bottle.

When cheap isn't the right answer

If you have cystic acne that scars, persistent rosacea, sudden pigment changes, or a mole that's altered in shape, size or color, stop shopping and see a GP or a dermatologist. A prescription tretinoin or an actual diagnosis will do in eight weeks what an eighty-pound serum won't do in eight months, and it'll cost you less.

Also, genuinely: some expensive products are nicer. Better texture, better smell, a bottle that doesn't leak in your bag. If the ritual of it means you actually stand at the sink every night instead of falling into bed in yesterday's mascara, the premium bought you consistency. That's worth something real.

But buy it knowing that's what you're paying for.

The whole routine, written out

Morning. Rinse or gentle cleanse. Moisturiser. Sunscreen, two finger-lengths, down the neck and the backs of the hands.

Night. Cleanse. Active, on the nights you're using it. Moisturiser over the top.

That's it. Six minutes, split across two ends of the day.

When your skin is irritated, the instinct is to add something. Do the opposite. Drop the active, keep the cleanser and the moisturiser, wait a week. Most flare-ups are self-inflicted by enthusiasm.

Put the four bottles where you'll see them and give it until March. Then look at a photo from today and decide whether anything you didn't buy would have made a difference.