Coffee-to-water ratio, water temperature, grind size, the bloom and the pouring order, plus which way to adjust when a cup tastes sour or bitter.
All you really need is a dripper, paper filters, a carafe or mug, a grinder and a kitchen scale. A kettle with a long, narrow spout makes it easier to control the stream, but a regular kettle is fine while you practice. Weighing the coffee and water the same way every time matters more than an expensive dripper. Put the carafe and dripper on the scale and watch the number as you pour, and you will have something concrete to go on when a cup does not taste right.
A weight ratio of about 1 part coffee to 15 to 17 parts water is a common starting point. Fifteen grams of coffee with around 240 grams of water makes one cup. The number is not a rule but a baseline for adjusting: if the cup feels weak, use less water; if it feels strong, use more. When changing the ratio, keep the coffee dose the same and change only the water, which makes comparisons easier. Keeping the same ratio when you scale up for a bigger mug keeps the taste from drifting.
Water a little below boiling, roughly 90 to 96°C, works well. Light roasts give up their flavor less easily and suit the hotter end; dark roasts turn bitter quickly and suit the cooler end. Without a thermometer, pouring boiled water into the kettle and letting it sit briefly lowers the temperature somewhat. Start with a medium grind, about like coarse sugar or coarse salt. Too fine and the water drains slowly, the brew drags on and turns bitter; too coarse and the water rushes through, leaving the cup sour and thin.
The sequence itself is simple. For the first few brews, run the scale and a timer together and check the numbers at each step.
Roasted coffee holds trapped carbon dioxide, so the grounds swell and bubble when water first hits them. If that gas stays, water cannot soak in evenly; it carves channels and runs out one side. Wetting the grounds with a little water first lets the gas escape, so the later pours reach all of the coffee evenly. Fresh beans swell a lot, while old beans barely rise. That means the bloom also gives you a rough read on freshness.
Change several things at once and you will not know what made the difference. Pick just one of these, change it, and compare with the next cup.
Using coffee ground days in advance lets the aroma fade quickly, and the cup tastes flat no matter how well you pour. Grinding right before brewing makes a big difference on its own. Pouring too hard at once and pushing the grounds up the filter wall is another common slip; the water then runs past the coffee instead of through it. Waiting for every last drop to leave the dripper also tends to add harsh notes at the end, so pulling the dripper once you reach your target weight gives a cleaner cup. Brushing out old grounds left in the grinder now and then also helps.
Coffee is mostly water, so the water's taste ends up in the cup. Clean water with no odor is enough; filtered tap water works fine. You are handling boiling water, so keep your wrist out of the steam when tilting the kettle, and in a home with children keep the dripper and carafe out of reach. If you are watching your caffeine, you can brew weaker with less coffee or use decaf beans with the same method.
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