It’s pretty simple.
Picture a white wall with no details in the distance. You don’t want to really see it just know it is there.
Everytime you think of something that distracts you, refocus on the wall, not directly but just make yourself aware of it.
The important thing is not to see details that will distract you.
I don’t need to use it often anymore but for a while it was the only way I could fall asleep.