Fermentation: where the beer is made
The yeast does the real brewing. Your job is to give it clean wort, enough oxygen at the start and a steady temperature.
Before you pitch
Cool the wort to the yeast supplier's recommended temperature, usually 18 to 20 C for ales, and aerate it by pouring or shaking, because yeast needs oxygen to multiply at the start. Then sprinkle or pour in the yeast.
Sanitation
Everything that touches the cooled wort must be sanitised: fermenter, lid, airlock, spoon, hydrometer. Cleaning removes dirt; sanitiser kills what is left. Most failed batches come from skipping this.
What you will see
Within 12 to 48 hours the airlock starts to bubble and a foam layer, the krausen, rises. Activity peaks over two or three days and slows after about a week. Do not judge by bubbles alone.
How to know it is done
Take a hydrometer reading when the airlock has slowed and again two or three days later. If the reading has not changed, fermentation is finished. Use the calculator to work out the alcohol from the original and final gravity.
Temperature
Too warm and the yeast makes harsh, solventy flavours. Too cold and it goes to sleep. A stick-on thermometer on the fermenter, and a spot out of the sun, solve most problems. Fermentation itself gives off heat, so the beer is warmer than the room.