Before going to draw a donkey what you are required to do is to observe a live donkey and observe every movement of it so that you can not miss a single movement of it and neither any feature. You can take a note also of the features of the donkey so that no confusion is being left in your mind. The next step you should follow is to establish a drawing paper on the easel and then try to draw it well the features that you have seen.
Initially you may get some problem but after one or two trials you will be able to draw it well. If you have the facility to draw the picture while looking at the live donkey, you can draw a better picture but that is only optional and not mandatory.