The firstly, you need install plugin WooCommerce Advanced Free Shipping and active it. http://prntscr.com/owfee2
Next, go to Dashboard > WooCommerce > Settings > Shipping > Advanced Free Shipping > set as image http://prntscr.com/owfb8p
Note: you need to create only one Free shipping rates with the format as image: http://prntscr.com/owfal3
Hope this helps!