Splines are curved sketches that you can use to create organic shapes.
There are two different types of splines:
- Fit point splines: The spline goes through each fit point smoothly.
- Control point splines: Every created point is a vertex of a polyline (invisible), and the app automatically creates a smoothed-out version of it.
Go to Sketch > Spline, and then choose the spline type: “Control” or “Fit.” You can further customize the required pencil pressure for the points. 
- You can start drawing a spline just as you would draw a line.
- Wherever you want to create a curve, press the screen with your Apple Pencil without lifting it up to create the spline points.
- To continue your curve, carefully release the pressure on your Apple Pencil (without lifting it) and keep drawing and adding more spline points.
Once you release your pencil from the screen, your sketch is done. - You can move or delete the spline points to adjust the curves.
Note: If you created a “Fit” type spline, then you can add more points to it by pushing down on your Apple Pencil along the curve.
See how to create splines in this video: