To book your slot for GRE, here's a step by step guide. Head to GRE. In the How To Register Tab, Click Register For The GRE Computer based Test register1. There are no time-limits to book the GRE slot. You can visit the ETS site and book any date and slot of your choice provided it is available; this can be 90 days. Step by step procedure for how to check for availability of test dates for GRE on selecting a date will give the time slots select the time and register for the test.