It is a bit involved but once you understand how Quest puts the pieces together, you'll see that it is really not that bad. CLICK HERE for the guide on how to do this.
This is just a matter of finding the "Print Assignment" menu command and then clicking on the Central Duplicating option.
After you have a specific assignment listing of questions showing, click on "Edit settings" in the rounded box at the top of the assignment listing. On the next page you will see a "Instructions to class (optional):" textbox. Enter your preamble to the assignment there. You can preview it before you save aslo.
While viewing the question listing for an assignment, find the question that you want to write a preamble for. Now click on the little pencil icon. You'll get a new window that allows you to adjust the points on the question, withdraw the question (like killing it), and create a preamble for the question.
First you must create the assignment TYPE in Quest and flag it as "manual" in the pulldown menu. Then you name the assignment and click OK. You'll then be prompted to upload your score file. The file must be a text file with the student's EID followed by white space (a tab or space will do) and then the numeric score. Then you upload it. Also type in the MAX score in the field provided so that Quest will calculate the percentages correctly.
This one is a bit involved, so go HERE to get the tutorial.