Set the sink on wood cleats before you install the counter.

Can you tell me more about this? Is it just doable or does it work well? Thanks

I usually use 2x4's and screw from the opposite side of the sink cabinet sides, which allows the versatility of being able to easily change the sink in the future without affecting the counter top.
Set the cleats and the sink in-place and level with the rough top prior to the measuring for the counter top, to remove any confusion.

in the kitchen forum you will find out a lot about undermounting sinks.

millions are installed throughout the whole world.
There is a little bit to learn, not a lot.