The most important skill for a barista is pulling a shot of espresso. As it might look easy, the checklist behind the pull is pretty specific. If any of these steps are not followed, your shot will be less than perfect.
The Grind should be like granulated sugar. If it is two course, your shot will pull fast which will be weak and often sour. If it is to fine, it will be slow (over-extracted) and will taste bitter and burnt.
The dose should be between 14 and 18 grams of coffee per 2 oz double shot.
The tamp should be done with your elbow at 90 degrees applying 30 pounds of even pressure. The result should be an even a polished shot. This can be accomplished with a slight twist of the tamper after applying pressure.
The pull should be 1 ounce per shot. Once you pull a 2 ounce double shot, stop the pull. The timing of this should always be between 20 -30 seconds. Anything under or over this will result in a over or under extracted shot. Sour, bitter, burnt or weak. A perfectly pulled shot should have a golden crema on the top.