You practice with pencil first.
Nothing anatomically correct or even realistic. Outline with pen to familiarize yourself with shape and form (sans pencil).
Make lines to loosen up. Link them into each other. Eventually they’ll form an image of some sort.
Walk around. Make random sketches of things you see. The grittier, the better (see Käthe Kollwitz). Use straight lines for shading. Crosshatching is like a magical therapy session.
I used to live in this area with tons of power towers. The sky was magenta in the summer, and the pavement was always wet.
These sketches are from an idea I played around with during the summer of freshman year.
Even if you scribble something out of place, you can trace and retrace until you draw the form (do you like puns) from the page with your pen.