First, I recommend educating yourself about the TOEFL test. Just look online to get the gist of what the test is all about.
Once you're familiar with the test, find a textbook that you like that includes practice tests. ETS makes The Official Guide to the TOEFL Test, which would do the job.
Give your private student a few practice tests and find out which areas he or she needs more practice. Then, focus on those areas during your lessons. At the end of each month, try the practice tests again to see if there is any improvement and also to see if there are other areas which need improvement.
I wouldn't use the TOEFL textbook as the main part of the lesson. It is a comprehensive test, so students need to improve their overall level of English and not just study for the test.