Stand and place your feet shoulder-width apart. You could possibly place one foot in front for more stability.
Grab the barbell underhand and place your hands about shoulder-width.
Look forward and keep your upper body straight.
Implementation & points of attention
Curl the barbell up to shoulder height.
Keep your elbows tight at your sides. It is very important that they do not move forward. You have to prevent it from becoming a half-front raise.
When the barbell is around shoulder height, hold it for a while and squeeze your biceps extra. This has a positive effect on your focus and muscle growth.
Then QUIETLY lower the barbell back. So often you see someone putting in a lot of effort to curl it up but then dropping it like a brick. It is precise with the eccentric part that you can focus on the biceps.
Make sure you get the movement out of your biceps (by keeping your elbows in the same position and not pulling it out of your back).