You can draw on an iPad with an Apple Pencil on the Procreate app, which you can use to sketch, paint, and illustrate.

This video shows a simple approach to creating doodle art in Procreate on an iPad. You'll learn how to use basic brushes, add colors, and build up fun compositions layer by layer. Suitable for ...

In particular, Procreate’s facility with digital photographs allowed me to analyze shapes of things I wanted to draw.