In the case of this particular scanning board, the alphabet is laid out in a grid, and broken down into rows and columns.
This scanning board has the same image on both sides. The communication partner holds the board in between themselves and the person with the communication disorder, so that they can both see it.
The person using the board has a pre-agreed upon signal which indicates that they are making a selection. (This can be a nod, a blink, eye movement, etc. )
The communication partner calls out or indicates each row. (“Is it on row A? E? I? O?)
The person with the speech disability indicates when they get to the row which contains the letter that they want.
Let's say they select the "O" row.
The communication partner then calls out the items in each column. Is it "O? P? Q? R? S?" The person indicates when the letter that they want is called out.
Video courtesy of Amy Román, SLP