Well, first figure out which depth you want and then start studying as soon as you can.
Now for some unsolicited advice. I took the Mechanical Systems and Materials test and this is what worked for me:
1. Gather up all your materials (formula sheets, books, prayers, MERM)
2. Take the NCEES 2008 Practice Exam (Timed)
3. Complete the following: 6MS Mech Breadth, 6MS Thermo Breadth, HVAC Breadth
4. Take the Lindeburg Practice Exam (Timed)
5. Retake the NCEES 2008 Practice Exam (Timed)
The reason I took the NCEES practice exam first was to get a feel for what I knew and what I didn't. For me, this gave me a road map of what I needed to focus on for the next few months. Feeling the pressure of a timed test will give you a sense of what you will need to prepare for.
Also, it may be of use to you to compile a 3 ring binder of all of your goto formulas and material and get used to using it. You'll come across a ton of great information as you study that you can apply to your professional career afterwards so why not compile it? Though I used the MERM primarily (tabbed like Shaggy), my binder had alot of "supplmental" stuff like the MERM Index, High Def Psych Charts, Cheat Sheets (Page of equations), Example problems, etc. that I feel increased my speed dramatically.
Didn't open any of the other 10 reference books that I brought.