Various approaches are used. At the very basic level all letters are made up of circles (whole or in part) and strokes. Sometimes teachers use the terms 'bats' and 'balls'. For the letter 'b' students are verbally instructed that the bat comes before the ball whilst for the letter 'd' the ball comes before the bat. But we know from brain research that visual presentation of material always wins out so using verbal language to create the correct neural pathway may not be the most effective.
With my students I have them put their hands in front of them, palms facing them and ask them to close their
A couple of weeks ago a friend informed me that her son (with dyslexia) had worked out his own way of knowing how to differentiate between 'b' and 'd'. She then showed me that when we start to make the sound /b/ our lips are tight together and in a straight line, i.e. the 'bat' is first when we say /b/. Similarly when we start to make the sound /d/ our lips are round and open, i.e. the 'ball' comes first when we say /d/. Since then I have learned that this technique is promoted by reading therapists worldwide. But be prepared. Just because they have learned to use the hand or mouth technique to differentiate between /b/ and /d/, children will need to check using their technique every time they encounter the sound or letter. Otherwise they will continue to guess - incorrectly.
Out of the mouths of babes .....