The four things you control
Grind: The biggest lever, and the one most people never touch. Finer grind means more surface area and faster extraction. If your coffee tastes sour, grind finer. If it tastes bitter, grind coarser. Change this before changing anything else.
Ratio: Somewhere around 1 gram of coffee to 16 or 17 grams of water is the standard starting point for filter brewing. A scale costs very little and does more for your coffee than almost any other purchase.
Water: Just off the boil, roughly 92 to 96 degrees Celsius, is the working range1. And since a cup is about 98 percent water, if your tap water tastes of chlorine, so will your coffee. Filtered water is a genuine upgrade.
Time: Each method has its own window, and it interacts with grind. If your brew is running far faster or slower than usual, the grind is usually why.
Freshness
Roasted coffee releases carbon dioxide for days after roasting, which is why we do not recommend brewing it the day it comes out of the roaster. It needs a short rest first. After that it is at its best for a few weeks, then slowly flattens.