You can change the shape of your plastic nibs to suit the type of drawing or lettering you want to do. You can shape the nib to your preferences. Our plastic nibs behave somewhat like feather quills. I say "somewhat" because the material is different and the flow of ink through the nib is different, so you should shape plastic nibs the same way you shape feather quill nibs.
You can cut and snip the tip to for the shape you want to use. We recommend you use small scissors and clippers designed for finger nails. Shaping finger nails is probably a better example of how you can approach shaping plastic nibs.
The two most common shapes are pointed and chisel tip, but you can modify those several ways and cut entirely different shapes.
You can recover from any mistakes by trimming the nib further back. You don't need to take off a lot of material. You just need to trim the edge that will lay down the ink trail.
Learn over time what shape works best for you, then keep your nibs trimmed for use. You aren't going to get hairlines, but you can draw thin lines to very thick and every line in between. You can draw hatch lines in close marching order, or swirl the nib in grand sweeps and gestures.