Hello Andreas,
when you unpack the sink, there is a gray, strongly self-adhesive rubber band included, which is attached to the edge of the underside. This also secures the sink to the countertop and seals it.
For the cutout, place the sink with the "good" side facing down on the countertop, trace around it, and subtract 1 cm all around from the traced outer dimensions.
Additionally, there are clips included that are screwed onto the edge of the countertop cutout and previously hooked onto the edge of the sink. These secure the countertop at the edge.
Furthermore, I applied aluminum tape to the edge of my wooden countertop cutout. You can also seal the edge with silicone. However, as the sink is delivered, this is not absolutely necessary.
But everything is described exactly like this in the instructions.
Hope that helps.
Best regards,
ALX