Sit on a bench with the back raised for best results. If standing, the need to stabilise the body makes the exercise less effective since more muscles are recruited and the brain has to concentrate on body stabilisation as well as good form.
Shoulders down and armpits closed. Elbows do not flare out.
Going for gold: as the weight is curled up, cock the wrists so the back of the hand is lower than the wrist joint.
At the top of the movement, turn the palms to face away, straighten the wrists and lower the weight more slowly than you lifted it. If the wrists droop, change to a lighter weight so you can keep them level.
For maximum effectiveness, all bicep and tricep training involves leaving the ego in the street shoes and sucking in humility for great gains. The Zottman curl demonstrated above gives excellent grip strength if needed. If doing weight exercises to look good, it increases bulk in the forearm. Bulging biceps and puny forearms look silly.