Click on the module you'd like to add the new assignment to and click "Create New" at the top right of the page.

Next, click on "Assignment"

Next, edit the Assignment Title, add your instructions and due date, etc. To link the assignment to a grade item that already exists, click on the box that says "Ungraded"

Then click on the phrase "In Grade Book" and finally "Edit or Link to Existing"

Click the radio button next to "Link to an existing grade item" 

Use the drop down menu to select the grade item and then click on the blue OK button.

Finally click Save and Close at the bottom of the New Assignment page.