Step 9 Select the line and hold Alt, whilst dragging it to create a duplicate. With the duplicate selected, choose Object > Transform > Rotate and set the value to 180 degrees. Click OK, and your result should be a mirrored version of the arc.
Step 10 For the next step, switch on Smart Guides (Cmd/Ctrl + U), and move the duplicated arc to match up with the ends of the original arc. The best way to do this in Selection tool mode (V) is by dragging the arc by either of its end anchor points, and lining it up to the corresponding end of the other arc.
After lining up the anchor points correctly, your cursor will change from black to white.
Select both the arcs, right click and choose Join. This joins the anchor points. The two arcs are now a shape.
Step 11 Select your new shape and fill it with black. Open the brush palette, if necessary (Window > Brushes or F5), and select New Brush (the button to the left of the trashcan). From the types that follow, choose Art Brush.
In the Art Brush Options dialog that opens, set the brush width to 25% and the Colourisation value to Tints. Hit OK.
We’ve finished making our highlights and shadows brush, so you can delete the merged arc shape from the canvas.