Start with the 787 Qualification Course overview (Training > Qualification). The first part of the course is 787 systems with CBTs and systems knowledge validation. You can start those CBTs 30 days prior and use the questions in the 787 Systems Guide to make sure you've got it for the validation. Then the Preflight Flow & QRC (Training > Train Guides/Other) along with the normals section of the FM for learning your flows.

The qual cause doesn't appear to be rushed, but if coming off the 777 and doing the differences course the only significant hurdle is the systems validation that is on day 5 vice day 8 in the qual course.

I wouldn't bother buying non-United guides as it isn't necessary and can make the process tougher IMO.

Disclaimer: My info is 4 years old.