There are 5 simple steps as below: 

Step 1: Open Chrome's developer tools with F12.

Step 2: Click the 'Toggle device toolbar' button to turn Device Mode on or off. (Icon is blue when Device Mode is on).

Step 3: (Optional) Select which mobile device you want to simulate with the 'Viewport controls'.

Step 5: Refresh the webpage (Ctrl+Shift+R)