Fill the rapidograph with ink specifically made for use with rapidographs (other inks will become problematic). Twist apart the different pieces of the rapidograph until you get to a translucent plastic cup. This is the bottle you fill with ink. After that, retrace your steps and put the rapidograph back together.
Hold the rapidograph up with the point end down for a minute or so to let the ink flow into the tip of the tool before you make a line. Once the ink begins to flow, use it like any other pen. Don't press down too hard on the paper, as this could cause the rapidograph's very delicate tip to bend.
To get the crispest, most precise lines, use a ruler, french curve or other drafting tool. Make sure the drafting tool has a small lip that sticks out and is raised a bit from the paper. This will prevent the ink from the rapidograph slipping under the ruler and causing unwanted bleed lines.
To store the rapidograph when you're not using it, replace the cap and stand it needle side up in a cup or pen holder. Don't rest it on its side, as this will cause the ink to flow unevenly, or tip side down, as this will cause ink to continue to spill out eventually.
Clean your rapidograph once you're done using it. If you use it on a daily basis, you won't need to clean it more than once a month or so. If you use it less often, you're going to have to clean it between uses to prevent ink still in the tip from drying. Take all the pieces apart, discard the excess ink and soak the individual pieces in the rapidograph cleaning solution.