You will need a Microsoft account, and an outlook.com or Hotmail account, to book an exam. You should navigate to the Learning dashboard at http://aka.ms/learningdashboard and sign in.
If you have not taken an exam before, you will need to create a Learning profile. It is best to do this ahead of booking your exam in case you have technical difficulties.
To book the exam, navigate to https://docs.microsoft.com/learn/certifications/exams/pl-200, click on Schedule exam, and follow the many steps. You can either take the exam in a test center, or you can take the exam online at home. If you plan to take the exam at home, make sure you read this page carefully and that you can meet the criteria: https://docs.microsoft.com/learn/certifications/online-exams.
Right, now for D-day; taking the exam.