Pre-infusion and the bloom
Wetting the coffee before you brew it properly is the same idea on an espresso machine and on a pour over. What it fixes, and how long it should last.
Extraction
Yield, strength and TDS: how to read a shot instead of guessing at what went wrong.
Articles
Espresso troubleshooting: what the shot is telling you
Read the symptom, find the likely cause, change one thing. A working order for fixing fast, choked, sour, bitter and channelling shots without guessing.
Why your espresso tastes sour
Sourness is almost always under-extraction, and under-extraction is almost always grind. Here is the order to change things in, and why that order matters.