Obtain objectives to be assessed. In order to be valid, assessments must reflect precisely the purpose of instruction. Create a table with one column detailing the types of questions that will be written as well as a column for how many of each type will be included. Use a hierarchy of thought such as "Bloom's Taxonomy" to develop types of questions.
Consider the examinee group. Adjust the level of difficulty to reflect their range of ability. Pose challenging but not frustrating questions. This will be a subjective judgement at first, which you can assess when practice test takers try out your test. Likewise, the level of reading must not be a barrier to the assessment of content. Write the assessment bearing in mind the examinees' independent reading level.
Write all test items precisely and succinctly. Extraneous information or scenarios can cause distraction and lead to misinformation. Keep out information that may lead to cues to the correct or incorrect answer in other questions.
Create the answer key or rubric needed to score the assessment before the exam is given to ensure consistency in grading. Find groups of people willing to proof the test, take the test and offer feedback prior to execution of the assessment. You will need to find people to represent various demographic questions, if possible.