To sign up for the assignment, you go to pearsonvue.com and select the 1Z0-865 assignment under non-proctored exams.
Then when it says to pay, you enter your voucher number. If everything goes well, you can download the assignment immediately.
You then have up to 6 months to do the assignment AND essay (part 3). Another voucher/payment is needed for essay. There is
discount till Dec 31 2015 if you done by then and register for essay.
Once inside the assignment screen, follow the steps accordingly.
Good luck.