Slanted curbstones - Horizontal

Hello,
I want to pave my driveway along a distance of about 15 meters with curbstones (length: 1 meter each). I have already tried several times with sand to get the whole thing properly aligned using a spirit level and guide string, but it just doesn’t work.
The problem is that the curbstones are supposed to be installed on an unevenly sloping hillside. For this, I try to place each new stone two centimeters lower than the previous one.
If I start at the top with a "perfectly" horizontal stone and place the following stones with equal spacing, the next stones start to rise on the side facing towards the lower end of the slope, as the spirit level shows. If the whole thing is supposed to be "perfectly" horizontal, then the gap between two stones becomes visibly uneven.
I have tried everything I can imagine, but it should also be noted that I am not a professional.
Could someone help? I would be very grateful.
Thank you very much!

I laid my deep boards with a guide string, spirit level, and [Splitbett]. Especially the [Splitbett] saved me a lot of trouble. Afterwards, I slapped concrete on the left and right.

Whether it holds, I don't know.