There may be easier methods than what I am going to explain. They may involve catch phrases like "confidence" or "just believe," but that is never really very helpful.

The first thing you need to do is learn to stop thinking of the visual field in a dream as solid or conforming to even the most basic principles of real life. To do this you start with simply attempting to rend or smudge the visual field. Start with an idea that you are looking at a chalk board. Put your fingers on a table top or wall and watch very closely. Feel as if it is a painting, not real. Now draw your fingers slowly down it and expect the visuals to get mangled and blurred like they would if you did this to a fresh oil painting. As soon as you can do this with your hand, practice doing this to an object simply by moving your hand slowly in front of the object. At this point you are not trying to reshape it, you just want to blur it or warp it. You are convincing your brain that this is just graphics like a painting that is still wet. Now move to doing this simply by visualizing the blurring effect. As soon as you can look at a candle and cause it to blur and melt simply by this method, it is only a short step to what you are after. You cause the object to blur and distort, then you see in your imagination/minds eye the new object form.

Hope that helps!