espresso preparation is the science of brewing finely ground coffee with hot water under high pressure. 

The variables of espresso preparation are dose, yield, and time.

DOse

When making espresso, start by deciding your dose: the amount of dry coffee grounds you intend to extract. Your espresso basket will be a factor in this choice—if it is marked on the rim (look closely) with a dose, start with that. If you’re unsure, start with 18 grams. To measure how much coffee you are dosing, place an empty portafilter on a scale and tare the weight. Then place it back on the scale after grinding and you’ll be measuring the weight of the coffee alone. Adjusting dose and another variable at the same time is like trying to focus a camera while walking forward and backward, so keep it consistent throughout this process. 

yield

This is the amount of espresso you’re making, by weight. Traditionally, this section is where you would hear terms like ristretto, normale, and luongo. These refer to ratios of dose/yield: 1:1, 1:2, and 1:3, respectively. For more, scroll down a bit further. For our purposes, we’re focusing on ratios in the normale range, so if you dose 18 grams, you’re shooting for a 36 gram yield. Measure yield by placing a scale on the drip tray below the group head before you begin pulling your shot. Place your vessel on the scale and tare it, then begin extracting your espresso. 

time

Adjust your shot time via your grind setting (finer=slower, coarser=faster) to produce an espresso that tastes best with the dose and yield you’ve selected, typically somewhere between 25 and 34 seconds. If you reach your desired yield too quickly and the espresso tastes sour, thin, and lacks sweetness, adjust the grind setting finer. Always purge a shot’s worth of coffee after making a change to push out any coffee grinded at the previous setting. The next shot should take a few more seconds and have more sweetness, better mouthfeel, and a more balanced acidity. Adjust the grind finer as needed; checking back with the scale to ensure your dose hasn’t changed. Eventually, the espresso will find its point of focus and the shot will be vivid and compelling. This is the sweet spot you’ve been searching for. It is possible to go too far and adjust the grind too fine, however. This shot will have lost focus, this time in the other direction. The flavor may be a bit dull in comparison to previous shots, the mouthfeel might feel thin, and the taste may turn bitter or harsh. If a finer grind adjustment has led to these diminishing returns, coarsen the grind and retrace your steps. The sweet spot shouldn’t be too far behind you.

What happens when espresso is brewed? 

Espresso is defined by pressure. Pressure shortens the extraction time radically in comparison to other brew methods and facilitates the release of less soluble compounds and rare flavor notes found in coffees’ oils and fines. This process begins with pre-infusion. The coffee receives a large burst of water and expands, filling gaps in the basket. As pressure builds, water seeks the path of least resistance and begins to burrow channels created in vulnerable positions in the coffee bed. Extraction begins to take place. Water extracts soluble compounds as pressure helps extract non-soluble compounds (oils). The bed of coffee becomes saturated as water passes through and a phenomenon referred to as fines migration takes place. Here, fragments of the coffee bean's tiny cell walls move through the bed and into the vessel below. The perfect symphony of time, pressure, dose, and grind produces a feast for the senses. Too long (over extraction), and the undesirable, less soluble, compounds make their way into the cup, like unsavory guests to a party that has gone past it’s hour.

ristretto vs normale vs luongo

Milk Preparation

1. Start with fresh, cold milk. Fill your steaming pitcher about half way.
2. Make sure your espresso machine's steam wand is clean and purged of stagnant water. 
3. Insert the steam wand into the pitcher and submerge the tip just under the surface of the milk, perhaps 1/4 of an inch. 
4. Activate the steam wand. The whirlpool/vortex should be in full spin. During this stage, air is being incorporated into the milk. The milk should expand in size about 33%.
5. Once the milk’s volume has increased by (about) 1/3, sink the tip of the steam wand into the milk—still maintaining the vortex, but allowing no more air to be incorporated into the milk. Use your free hand to feel the side of the pitcher throughout this stage. As soon as the pitcher becomes too hot to touch for more than one second, cut off the steam wand.
6. Hit the bottom of the pitcher against the counter to bump out any bubbles. Then spin the milk in the pitcher until it develops a chrome-like sheen on its surface. The milk should have a sweetness in smell and taste. If it tastes burnt, cut off the steam wand earlier next time. 
7. Keep practicing! Microfoam requires a lot of time and effort to master, and recurring practice to retain mastery. 

Cleaning Your Espresso Machine:

More on Fines Migration

‘Fines’ are microscopic particles of coffee cell walls that contribute to the flavor and mouth-feel of espresso. They move with water through the coffee bed and most of them wash into the espresso in the first 15 seconds of extraction. If the coffee is ground too coarse (usually in conjunction with a very high dose in order to achieve the ideal time & volume measurements), it is EASIER for the fines to move through the large pieces of ground coffee—so you end up with more in your espresso, leading to a sensation of heavy body and some bitterness. If the coffee is ground too fine, less fines migrate through the coffee bed-because the grind packs together more tightly. If the coffee is too coarse (usually in conjunction with a particularly low dose), a smaller amount of fines are in the espresso and the espresso can have a thin, lackluster mouth feel.