Hammer work
Swing true, move metal, leave no cold shuts.
Swing with your shoulder, aim with your eye
The hammer face does the work, but the body drives it. Keep your elbow loose, strike on the rebound, and let the anvil do half the job — a dead anvil is a dead arm.
The basics
grip hold low on the handle, not up by the head stance feet shoulder-width, anvil at knuckle height strike face flat, never edge-on rhythm a 2-lb hammer at 80 beats a minute is plenty
What you're trying to do
Every strike either moves metal or consolidates it. Draw out to lengthen, upset to thicken, fuller to neck it down. The cold shut — a fold you hammer back into itself — is the mark of a beginner, and it shows up in the grain as a crack.