Sketching horns, huh? Totally rad! Let's break it down. You can start by outlining the base of the horn near the head - this should look like a curved rectangle. Imagine the horn as a cone extending from that. Draw lightly at first, it's easier to make corrections that way.
Now start sketching the lines that will form the curvature of the horn, starting wide at the base, coming in for the curve before heading back out for the tip. Add a final touch with some shading and lines to give texture and depth.