In the weeks after buying the voucher, you'll receive an invitation to take the exam. (Or multiple invitations for your team, if you bought multiple vouchers.)
Using that link, you can visit the exam site when you're ready. You'll be asked to provide some personal details.
When the exam starts, you'll get a number of questions. You'll need to answer them in the time allotted. You can skip questions if you like. At the end, you'll be able to review your answers before finally submitting them.
If you passed, you'll receive a certificate after a couple of days, and you will be a Certified Laravel Developer.