If you're going to use any plastic devices like triangles, curves, etc. with a technical pen:
Put some masking tape (small pieces) on the back side of the triangle or curve, spaced around the edges and about 1/8 inch in from the edges, to lift it off the paper surface just a little. If you don't do this, there's a good chance the ink will bleed under the triangle or curve and make a mess of your lines.
Begin your line with the pen just above the paper surface so the ink, not the pen tip, will touch the paper. Then draw the line with the pen tip still above the paper surface (just a hair) allowing the ink to flow from the pen onto the paper. That should help you to get cleaner lines.
Make sure there's no oil from your fingers on the paper. Otherwise, the oil will resist the ink and cause you to have ugly broken lines. If you're not concerned about health hazards, you can clean the paper with Bestine (available in art supply stores) on a tissue. Unless you keep your hands off the paper, you'll need to do this frequently. **Bestine is flammable, so don't smoke or allow the fumes to get near fire of any kind.**
If you're concerned about health hazards, just cover your paper with another piece of paper, only exposing the area you're inking and move the protective paper as needed to always keep your hands off the paper.
Use a hair dryer to quickly dry the ink and avoid smudging it when it's wet.
For a more extensive tutorial on maintaining a Rapidograph pen, and using it, see: