When building a quiz you have the option to add a set dedicated instructions at the beginning.
To do this, start by creating a quiz. Then in the left hand column you will find a tab labelled 'Instructions' click this.
Next simply enable the instructions module by clicking the check box.
Once you have done this you can begin populating the instructions module. Simply fill out the title and populate the body with the main instructions you wish to inform the takers of your test about.
Once you are happy with your instructions simply click 'Save' in the top right corner.
Updates coming to instructions: The ability to set timed instructions is in the works and will be released soon.