How do I level countertop before installing subway tile on backsplash?

When I started to install subway tile on backsplash, I realized the counter top wasn't level. Yes, someone dropped the ball on the top install!
The longest top is 11'. They didn't put a board on the wall to support the countertop. The board was added later but it is only level for 5'.
When I used a laser level, the top is down on the opposite end.
How can I level the whole top before I start to install tile?

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