How do I dial in a new bag of coffee?

Dialing in is a search, and searches go faster when you hold most variables still. Lock the dose to your basket rating, usually 18 g, and lock the ratio at 1:2, so 36 g out. Those two numbers do not move during the first phase. The only thing you are changing is grind size, and the only thing you are watching is time.

Pull the first shot at whatever setting you used for the last coffee. If it finishes well under 25 seconds, go finer. If it drags past 35 seconds, go coarser. Move in meaningful steps at first: a lot of people creep by one click at a time and burn half a bag getting nowhere. Two or three steps on a stepless collar is a reasonable first move, then halve the step size as you close in.

Once the time is in range, stop chasing seconds and start tasting. Sharp, thin or salty means under-extracted, so go a little finer or raise temperature one or two degrees. Drying, ashy or harsh means over-extracted, so go coarser or drop temperature. If the shot is balanced but boring, try lengthening the ratio to 1:2.5, which often finds sweetness in lighter roasts that a 1:2 leaves behind.

Freshness moves the target, which is why the same bag needs re-dialing. Beans four days off roast are full of CO2, which disrupts the puck and can make a shot behave erratically. Five to fourteen days is the reliable espresso window. As the bag ages past three weeks, degassed coffee flows faster, so you will drift finer over the life of the bag. That is expected, not a mistake.

Keep a note. Coffee name, roast date, grind setting, dose, yield, time, temperature and one line about how it tasted. It sounds fussy for about a week and then it saves you an entire bag every time a roaster you like sends you the same coffee again. This is the least glamorous advice on this board and the one that most reliably improves people's coffee.