I always advise a proper study plan to anybody sitting for an exam. Know your current knowledge level and plan accordingly. If you are thinking of taking this exam, you should be already at the CCNP Security or CCSP level, and therefore your skills should already be well-honed in order to book this exam.
Check your diary and book the exam for a date as long in advance as you think you need to study. I would always recommend at least a month in order to slowly work up to a crescendo in your study. Get the reading material and formulate a plan.
Start by reading the books from cover to cover, but also complement this study of the theory with hands-on practice. I am a very practical person, and I find that trying out the configurations really helps with my memory retention. Configuring the different types of VPNs on real Cisco hardware with all the different settings will really help the configuration commands and options sink into your memory.
Use the questions in the study books in order to test your knowledge and also look online for any sample exam questions you can find in order to check your knowledge. Also, if you can, speak to somebody who has passed this exam either in person or online and get them to ask you questions to gauge your understanding of the subject matter.
This is a difficult exam, and you need to dedicate a good amount of study time to it. It is a worthwhile exam to pass, especially if you are going for the CCIE; the CCIE certification can really change your life. It is important to realize this and make the decision to go for it with all of your abilities.