when i use the boolean modifier, nothing happens
You need to explain - ideally with images or a screen recording - step by step what you are doing and what doesn’t work if you expect any help.
You may want to read this: https://cgcookie.com/community/21510-advice-for-effective-q-a
This is as far as I got. But when I hide the cylinders nothing has happend and there are no holes
Notice the boolean modifier's icon is in red. That's because it's not doing anything since you haven't given it a target object. It is always the best of ideas to start over when you feel lost. If you think you missed a step and are now struggling, start over the lesson or the whole course. You'll fill the holes left in your first viewing.
