Look over the associated sections in the Instructor Manual. This is a good time to read chapter 2 in the Encyclopedia. You'll be reviewing parts of the Open Water Diver Manual, Adventures in Diving Manual, Rescue Diver Manual, and the Encyclopedia. Use section 4 of the Workbook to guide you through this one. Review both of the booklets: RDP, eRDPml and master the use of both. Read chapter 5 in the Encyclopedia, starting from Decompression Models. There are a couple of references to the Open Water Diver Manual, Adventures in Diving and the Deep Diver manual, so skim those as directed in the Workbook. Read chapter 2 in the Rescue Diver Manual. Read chapter 1 and chapter 3 in the Encyclopedia. Read chapter 4 of the Rescue Diver Manual. Read chapter 5 in the Encyclopedia, up to Decompression Models. Make notes for yourself, defining the gas laws and formulas in the simplest terms you understand so you can quickly review your notes right before the exam. When you have a question that starts at one depth and asks you to solve for another depth, always take it to the surface, then back down. Understand the gas laws and know how to use the formulas. Read chapter 4 in the Encyclopedia and then complete section 1 in the Workbook.

This is the one that most people sweat about and for most it is the hardest test.

You can always ask your Instructor for a class session on the topic! If you missed it, go back and study the Resource for the Objective and try again until you understand. Now go back and complete the Diving Knowledge Workbook, re-reading and studying the Resources for the Objective. Don't be too obsessed with picking up everything just yet. Some Instructors will let you take each exam separately rather than doing all 8 at once, so ask if you can do this. Do this for each exam you're focusing on, rather than trying to do it all at once. Read the sections in the manuals and Encyclopedia to get an overview of what will be covered. Since this topic comes up quite a bit, I thought I'd post a quick snippet of what to study for each of the 8 exams:

Divemaster exam answer key