This is not an Affinity Designer tutorial. If that's what you are looking for, keep searching. 🙂 My goal is merely to highlight the usefulness of iPad and Affinity Designer (or any vector drawing program).
What you do in vector drawing programs, is draw different shapes and combine them into objects. Rectangles, circles, triangles, lines, and so on. I usually start out drawing shapes by hand, using the different tools. Then I set the exact measurements I want them to have. Often I let one millimeter equal one pixel or some multiple of that. If I want to draw a pole that is 170 centimeters high, I set the height to 1700 pixels. By using real-life measurements, I will see if something doesn't fit or add up. One crucial aspect was to see that it wouldn’t be low enough for tall people to smash their heads in. Another was to make sure it didn’t block the view from the living room windows.
In the example above, I have drawn a construction to mount solar panels on. It will also act as a sun and rain cover above our terrace. The whole drawing is made up of rectangles only. Some of them are angled, but most are straight. If you know from the beginning what you are going to draw, the process goes swiftly. In this particular case, I only had an idea of what I wanted it to look like, and I went through many different designs as I iterated the construction when I noticed things that wouldn’t work. I also had to make adjustments when I got the real measurements of available beams and other construction parts.
Even for someone with no ability to draw a cat or face, this process makes it easy to create useful drawings. After the design was finished, I could show it to the solar panel company and ask for feedback.