I cannot find a tutorial on this… and I did it without knowing how.
I need to have a credit-card-sized box of about 10mm thick, with a circular bump in the middle.
I had an extruded rectangle of the correct shape and a circle drawn in the sketch which I extended into a hole. But somehow, I was able to do something which made the rectangle taper off without seemingly using the taper tool on the edge of the circle, but instead made the rectangle thinner on the edges, but kept the depth of the hole. It looked somewhat like a mountain or a volcano.
Not sure how to describe this, but a volcano-shape seems to represent it.
Any idea what I did? I did have two circles in the sketch at first (concentric).