The safe drop button is located in the "Cash Drawer" section of the POS tab.

Making a safe drop is nearly identical to the process of making a cash transfer.  To make a safe drop, enter in the quantity (NOT the dollar amount) of cash being put in the safe and select "Make Safe Drop."  If the amount moved to the safe brings the drawer's total below its base amount you will need to transfer cash back into the drawer when closing it out at the end of the day.