Use your attention, your consciousness. This exercise is not about thinking.
Visualization and imagination are powers of the consciousness. In the same way that you can imagine a banana in your head when someone mentions one, in the same manner we learn to focus, concentrate, and imagine the nadis or energetic channels in our spine. This is done in a purposeful and prolonged way by the consciousness.
Remember this phrase, "Wherever we direct attention, we expend creative energy."
Use your attention, willpower, the power of the consciousness, to raise the energy up the spine. This is not a mental exercise.
When using your will, you combine this with imagination, seeing the energies, in order to transmute.
In this manner you will begin to sense the energies, but not as sensations. It is something else in the consciousness that you must become acquainted with through experimentation.
Be relaxed and patient, since it takes training to learn how to teach the body how to do this!