To book your slot, you have to first create a "My GRE account". You then have to go to this official web page:
You can take the GRE five times within a 12-month period. For example, if you take the test for the first time in the month of June of a particular year, you can take the test only 5 times before June of the next year.
Further, there has to be a minimum gap of 20 calendar days between tests. For example, if you take the test on the 1st of June 2014, you can again take the test only on the 22nd of June 2014.