see the Sun website.
You buy an exam voucher from Sun, then with that voucher go (online, phone, or visit them) to Prometric (they're the company handling the actual exams).
There you make an appointment for a specific time and place to visit one of their offices to take the exam.
The exam itself is performed on a computer in what is essentially a classroom. The supervisor will take your voucher and check whether the code on it matches the code on their appointment sheet. They'll also check your ID (you need 2 pieces of ID, one of which must be a government issued photo ID like a passport) to see whether you're indeed the person who should take the exam.
Then they log you in on the assigned computer.
After a short explanation of how the software works and a short survey you get presented with the first question and the timer starts.
When you're done you confirm that to the software and it calculates your result. You will get a scoresheet on your screen which will also be printed for you. The supervisor will give you the scoresheet and you go home.
If you passed you will get your certification paperwork and stuff in the mail at the address you supplied when signing up a few weeks later.