Step one:
First draw a curve like this.
Step 2:
Draw arcs on both sides to connect them, leaving some gaps.
Step 3:
At the notch, draw some short lines as the texture of the feathers.
Step 4:
Next, draw the shape of the pen tip to give it a pointy feel.
Step 5:
Then you can also draw a line as handwriting.
Step 6:
Finally, apply a nice color and the quill is ready.