The meditation technique for the actual practice of tattwa shuddhi - the inner purification is described on a few pages near the end of the book. The rest of the book goes into some detail over different tattwas.
What I love the most about tantra is that it is practical and personally I am more interested in technique itself rather than in lengthy descriptions or theories, as I consider the personal experience of greatest importance.
The technique in essence entails dissolving the elements, from the grossest to the subtlest - within one's body - into nothingness, and then reassembling them.
Previous to working with this method, I have worked with Taoist method of purification of the elements within one's being, and with western Hermetic system (which even though derived from Indian yoga, is not the same as described in this book. The techniques of purification of the elements are all different and I have found them all valuable, as they all bring about different experiences.
The technique described in this book is again adapted from the original, which according to the author is more challenging to understand, and yet I think it would be good to at least have a look at what are the original recommendations and how is it to be practiced. One of those recommendations seems to entail focusing on one tattwa for 2 days before moving onto the next, and I think it is a good idea.
Apart from the technique for purification of the elements, and the suggested light died - preferably diet of fruits and steamed vegetables - during the practice, there are further recommendations that may not be practical for people living and working in the west (like shaving of the head) and using oil lamps, and some other details that one may need to adapt. The author offers variations for the practice where one may decide to engage in more intense practice, or to do practice at a lighter pace over the months or even over the years.