How to deduct Home Office Expenses due to COVID-19(T777S)?

Use "Employment Expenses" page. At that page, add a form T777s to claim this deduction.
On T777s, Option 1 ($2/day) does not need any supporting document while option 2 (based on actual expenses) needs employer signed T2200s and receipts to support the claim.

For more information, here is CRA guide on this topic.

If you are in Simple view:

Click on "Employment Expenses" on the left ==> select T777S as form type ==> click Add Form button to continue.

If you are in expert view:

Click on "Employment Expenses" under "Expenses & Deductions" ==> select T777S as form type ==> click Add Form button to continue.