Hi PaulDean,
Welcome to Affinity Forums :)
This is due to a bug that prevents the boolean operation Add to work properly (already logged).
To "fix" your project do the following:
- select the shape on the top (the large one)
- change to the Node tool and drag a marquee around all nodes of the shape
- click on the Smooth button in the Convert section on the context toolbar
You can now perform the add operation as indicated in the tutorial, selecting both shapes and pressing and holding ⌥ (option/alt) while clicking on the add icon.
The result should be correct.
