That is a great tutorial Ascania, just the job. I find the extrude surface tool a little tricky to get to grips with.
You just have to keep an eye on the red and green lines. Watch where my mouse pointer goes when I extrude. First it looks like a mess but as soon as I found where the lines were (because Hex likes to put them in semi-random places) I was able to get a good judgement of how the extrusion went. Also, you have to be gentle on the mouse while extruding. Large jerky movements are not the way to a solution.
As for the “just the job” bit. I utterly loathe the tutorial series that every single time over and over again explain to you how to use each tool at its default function. Be clear about what tool you use, give a short explanation when you do something unusual with it but for the rest - there are a thousand others and the manual that have already explained what the tool does. You don’t need to do it again.
And the fact that the Quicktime Player Screenrecording function is not all that good and makes everything laggy has nothing to do with it of course.