When you are talking about making a symbol system for someone who is severely cognitively impaired (the term mental retardation is no longer used within the education system) you are most often talking about using a series of pictures or images that the student can use to communicate their needs and wants in the absent of verbal language. The child will learn to point to the image to communicate what they would like to have. For example, a child may have a flip book of images that they carry with them. If they want a glass of water they will flip to the picture of a water glass and point to it and show it to their care giver, indicating they are thirsty and would like water. If the child would like to watch a movie they may point to a picture of a DVD to indicate that they would like to watch a movie.
It is important to note that the use if flip books or picture books require repetitive practice and operant conditioning in order for the child to make the connection between the picture being presented and their need being met.