When I was in the fourth or fifth grade I was the blackboard monitor
Basically my job at the end of each day was to take a bluish gray speckled enamel bowl the size of a big flat soup bowl.
I would fill it with water and soak an old blackboard eraser in it. Then squeeze out the water and wipe the blackboard clean. All during the day as chalk in the board was erased with a dry eraser it left a white dust behind. The wet eraser cleaned off all of the dust but you could not write on it again until it was dry. Thus it was cleaned only at the end of the day. The position of black board monitor was awarded to a student as an honor of excellent behavior. To me it was fun to do. I especially enjoyed the color of the blackboard going from dusty gray to an immediate wet black clean surface. I did careful vertical strokes that created an interesting pattern across the entire width of the board.
For this meditation the detail of visualizing cleaning the blackboard is important.
When you stop looking at a flight of steps after you have walked down you have dropped the physical image of them as well as the conscious awareness of them. You are down the steps and it is fine and gone. After looking at s sunset it may be gone visually but consciously you may hold on for a while.
Any negative experience or a worry may tend to linger in consciousness even after the event is over. It may linger in conscious even though you are trying to not think about it.
Erase the consciousness of this unwanted thought with your very own blackboard consciousness eraser.
With eyes open or closed and gazing up and to the right visualize the thought you want to erase. Write it out on the black board several times.
Then visualize a really wet soaked eraser and clean the caulked written words off the board.
Use even vertical strokes and notice the immediate change of color on the board. It goes from dry dusty gray to wet dark almost black. You can even wash it a second or third time.
As you finish clean the wet eraser and the ceramic bowl. You thoughts will now be clear and bright.