Creating an answer key depends on how the test is administered. If it is a multiple choice one, use a Scantron as the answer key. If there are several versions of the test, make an answer key for each one. Fill in the field labelled "Last name" with the instructor's last name and the one labelled "First name" with "key." For "Course number," fill in the course code and section, without blank spaces.
Fill in "Date of exam" with he date of the examination. Leave "Student number" and "Sex" blank. For "Exam version no." write the number of the test version the key is for. Don't use "01", "02" and so forth, or "A", "B". Rather, number them from 1 to 16. You might have to check whether students filled in the version number properly, because if they make a mistake, it can affect their marks. Once all of this is done, fill in the letter corresponding to the right answer for each question.
You should know when setting to prepare the exam itself that the Scantron can work with no more than 250 multiple choice questions. There are five possible answers, though it is recommended that only 4 options are used.
The Scantron system can also accommodate for True or False questions. For these, assign option A as true and option B as false.