Are you looking for this to be across your site, or just in one course?
You could create an Assignment 'template' that contains your basic information and then it can be duplicated each time you want to create a new assignment so you don't have to start from scratch.
Another manual option is to use the Generico plugin. You can program the instructions into one of the button templates to be the instructions you want to see in each assignment - then you can insert them into each new assignment with the click of a button.
That's all I can think of at the moment. Good luck!