Research the field. Look around at the metaphysical books marketplace. See where the gaps are. Now sit down and evaluate your own knowledge. Find where the gaps in the marketplace overlap with your own knowledge, experience and interests.
Select and outline your topic. Make a final decision on your topic. Sit down and list everything that you know about it. Now put your ideas into a logical format. Make the outline cumulative. That is, make it so ideas grow out of the ones before them.
Involve the reader. Metaphysical teachers of the past preferred teaching lessons to students rather than simply imparting information. There are ways to do this with the printed word. Ask the reader questions. Have exercises at the end of each chapter designed to drive home the spiritual lesson. This will go a long way toward helping the reader assimilate the knowledge you wish to impart.
Avoid or explain jargon. Every spiritual tradition is filled with accumulated, exotic-sounding jargon. It is best to avoid using such words. Better to use more contemporary words, such as "breath" for "prana." However, in some places the jargon word is better than a contemporary verson. For example, "cakra" is more succinct than "psychic energy center." Ultimately, the call is yours. Always err on the side of clarity.
Have an editor. It's very important to have another person (or three or four) look at your metaphysical book after it is finished. This will help you to clarify your ideas. When speaking on the ineffable it is important to make sure you communicate clearly. Having others tell you where you did not will make your book much more readable.