I suggest downloading the Program Guide, which includes the agreement and directions for enrolling. There's also a specific guide for the PowerShell 3.0 Toolmaking examination, which you should read prior to paying.
Once you've paid, and sent in the necessary signed paperwork, you'll get your exam info via e-mail. You can log in at any time to download your exam scenario and begin working. From the time of your first login, the clock starts ticking and you have 24 hours to upload your results. After uploading your results, you'll hear back within 5 business days - these are graded by a human, not a machine, so be patient.