Click Home > Feature > Chamfer on the ribbon.
On the Chamfer dialog, select Cross Section > Asymmetric .
Set the Distance 1 and Distance 2 values to 10 and 20 , respectively.
Click on the side edge of the selected face, as shown in figure.
Click Apply on the dialog.
Click on the rare side edge and click the Reverse Direction button.
Click OK to apply the chamfer.