Explain why you haven't taken the GRE prior to applying for a doctorate program. In doing so, be honest and succinct, and avoid negative language. The point of your GRE waiver justification is to explain not only why you haven't taken the GRE, but why your other experiences are acceptable substitutes. Don't linger too long on this section, as you want to get into the highlights of your academic career as quickly as possible.
Highlight your professional and educational achievements, preferably ones that would coincide with information provided by the GRE. If the application asks for examples of previous work, this is a good time to provide something that highlights the three sections of the GRE: verbal, quantitative and analytical writing. It might be a good idea to include previous work anyway, unless the application specifically says not to include anything but what is requested.
Sell yourself. Don't forget that the overall goal of a GRE justification waiver, in addition to all other materials requested by the school in question, is to get yourself into a doctoral program. This is no time to be humble; use strong, positive language when describing yourself, but do so without appearing boastful.
Edit your waiver as thoroughly as you can before you send it with your application materials. It's usually a good idea to have someone else proofread your waiver not only for spelling and grammar, but to ensure that it's professional and coherent. Another person will also be able to tell you if your writing is positive about your achievements without bragging. Another person's perspective can be invaluable for reviewing the content of your writing.