Just do your preview once. It should not take you more then 3-5 minutes to do. Just do what the book says!
About trigger words, you can wait until your review is done or to write down a few words during your 3-5 minutes. Trigger words give you a chance to see what is important in the book and also gives you a chance to start forming questions that you want answers too!
Get the self study course ASAP! Paul's tape seven going through the parts of the system on a book of your choice is the best model on how to Photoread