Let's talk about espresso. As a Starbucks barista who moved from a core store (most Starbucks stores you are used to) to a reserve bar (elevated Starbucks store) I moved from an automatic mastrena to a classic black eagle espresso machine. I no longer press a button and have the machine do all the work for me. I now control the grind, dosing, tamping, and brewing of the espresso. Because of this I wanted to delve deeper in to the art of pulling espresso.
The basic idea of espresso is water forced through finely ground coffee at about 9 bars or 130 pounds of pressure. There are three layers in the final espresso that Starbucks recognizes and looks for; the heart, body, and crema. The heart is the darkest layer at the bottom of the espresso. The body is a lighter brown middle section. And the crema is the lightest brown foam that naturally forms as the espresso pulls. The crema is what the specialty coffee world focuses on because this layer of espresso is what gives you the contrast necessary for pouring latte art.
We begin our espresso journey by dosing the correct amount of ground coffee into our portafilter (the metal basket and handle of an espresso machine). According to the Specialty Coffee Association, the ideal weight of espresso is between 14-18g of coffee. All though there is a growing movement within the community to dose between 18-20g of coffee. It is also important to 
have the correct grind at this stage. Depending on the grinder you use, you may have to trouble shoot until you find the perfect combination. Generally speaking you want a fine enough grind that the entire espresso pulling process takes 20-30 seconds and the resulting weight is 2 oz. of liquid.
Next you tamp the espresso. The term 'tamping' means to compress the coffee grounds. This stage is important because if not done properly the resulting espresso may taste sour or bitter. 
The water must be able to evenly pass through the coffee for the optimal extraction. When tamping you are pushing down with about 20 pounds of force which is not as much as you might think. If you tamp too hard the puck will become too compact which will interrupt the water's path through it. If you do not tamp hard enough the air will not be pushed out of the puck and the water will not flow evenly through the puck. In episode 25 on the podcast "Stone Creek Coffee", the two hosts talk about using a bathroom scale to measure just how much (or how little) 20 pounds of pressure is. 
Of course, you should first wrap up the scale or your tamp to avoid any gross cross-contamination. Another important aspect of the tamp is hand position. You are looking for your thumb to be pointed down towards your portafilter and your wrist to be straight. This is going to help you protect your wrists from repetitive injuries.
Once you are finished tamping, your puck should be smooth and flat. You can then insert your portafilter into your espresso machine making sure to lock it into place. 
A pro-tip is to turn on your machine for a few seconds before inserting the portafilter to warm up the group head and clear any remaining grounds from previous espresso. With the portafilter inserted you are ready to brew your espresso!
Depending on your machine you may have one that automatically stops after a certain amount of water has been dispersed, or you may have to stop it yourself. My home espresso machine, the breville duo-temp pro, requires you to stop the water yourself. 
Another important thing to note is the temperature of the water. You are looking for a water temperature of between 195-205 degrees Fahrenheit or 90-95 degrees Celsius. When pulling espresso, I start a stop watch and pull directly into a cup on top of a scale. I am looking for my espresso to reach the desired 2 oz. in 20-30 seconds.
If your espresso is not reaching 2 oz. in 20-30 seconds you may want to double check your grind. It is possible the grind is too fine which is not allowing the water to pass through quickly enough. If the espresso is reaching 2 oz. sooner than 20 seconds your grind may be too coarse allowing the water to pass much too quickly through the puck.
This process may seem daunting, but I promise the perfect shot of espresso makes it all worthwhile.