This is a super helpful tool that is not immediately obvious when you start using the app. To draw a perfectly straight line, simply draw a line (however wobbly), and when you reach the end, hold your Pencil there for a few seconds and the line will automatically become straight. You can then move your Pencil to change the length or angle of your line.
To draw a perfectly even circle, draw a wobbly circle and again hold your Pencil at the end for a few seconds, and it will automatically link up and create a circle that is even all the way round.
At the top, you will then see an “Edit shape” button appear, where you can choose to make a perfect circle, ellipse, quadrilateral, rectangle or polyline. This tool is great if you want to do some drawing that is geometric.