Go to the Settings app on your iOS device, then navigate to Safari > Advanced, and toggle on the Web Inspector switch
Open Safari on you iOS device and navigate to the web page you want to inspect
Connect your iOS device to your mac via USB
Open Safari on your mac you connected the device to and select Safari > Preferences from the menu bar
Then in Preferences click on the Advanced tab and at the bottom tick Show Develop menu in menu bar
From the menu bar select Develop, you should see a list of devices conned to your mac, click on the device you want to inspect. This should open Safari Dev Tools on your mac for the webpage open in Safari on your device.