Thanks. I learned it from an Udemy course [0]. Took just a couple of weeks to pick up. Regarding overlays, Sketchbook supports the idea of layers. I simply put different elements in the illustration in different layers. Sketchbook gives me PSD files that can be imported in GIMP. I then export many PNG files by progressively selecting more layers in GIMP. These PNG files go into Beamer like this:

    \begin{figure}
        \includegraphics<2>[width=0.35\textwidth]{images/shop/1.png}%
        \includegraphics<3>[width=0.35\textwidth]{images/shop/2.png}%
    \end{figure}

The % sign is important and it maintains the correct positioning of the images.

[0] https://www.udemy.com/course/drawing-for-trainers-leaders-an...