You will fill out the T2125 - Self-Employment form -Part 4 - line 8811. (Direct entry)

Downloaded version - Go to Formsview down at the bottom of the page, click on forms and do a form lookup for "T2125" form open, go to Part 4 -Expenses - line 8811 which is for Office Stationary & Supplies.  

Keep all your receipts for furture reference if CRA requests them.