This is a great question. The transition you talk about, dwi koobi (back stance) to ahp koobi (forward stance) is one of the key motions in several WTF forms. Although you may be able to put yourself in the stance one moment at a time, moving from one to the next is difficult. When you watch someone, like your instructor, seemingly move effortlessly you might be surprised at what he is doing to get there.
Here are a couple of pointers:
Control your height. When you control your height you control your center of gravity (balance), transitions become much more natural and look nice too.
Move like you are in a narrow hallway. If you imagine that you are in a narrow hallway as you move your body tends to use more efficient motion; arms stay in and you do not try to take up as much space. That is a big key in transition, don’t try to use more space than you need or your body will have too much momentum and at the end point you will have to use energy to stop yourself; you should be stepping not throwing your weight.
Be very aware of your posture. Your back needs to be straight and not moving around. It should kind of feel like you are wearing a scuba diving weight belt. If you lean in any direction the weight pulls you off course.
In this specific instance, from back stance, keep your height the same and keep your head looking the same direction. Imagine that you are in a tight hallway and slightly lift your front foot, push with your back leg and have it go from the 90 degree to the 30 degree as you move your hips forward, not your foot. Your foot will move forward but you are not trying to move it, it moves because your hips do. These actions happen simultaneously.
Once you practice this type of thinking you will go from a robotic type form to a much more organic looking motion.
ron