First of all explain to the students that they are going to do a picture dictation, that you are going to describe a picture to them and that all they have to do is simply listen and draw what they hear you describe.
You then describe a simple and easy-to-draw picture to them and they draw it. To help you with your first picture dictation you can use the picture on the accompanying worksheet (download below) and the description beneath it as a guide. It is a very simple picture for a low-level beginner's class but this kind of activity can be adapted to any level of student. Simply change the content of the picture accordingly.
When you are describing the picture it is best to describe one object at a time slowly and to repeat each description two or three times.
Make sure you give students enough time to finish drawing one object before you move onto the next object and it is a good idea to walk around and look at the students' drawings as they are drawing them so that you can see how well they are understanding your descriptions and then you can adjust them accordingly and give them any support they need.