Open the webpage on its own window, with no other tabs.
Make sure you’re not in full-screen mode.
Hide the Favorites Bar.
Hide the Tab Bar.
Press cmd+shift+4 on your Mac. (Your mouse cursor will become a cross.)
Press the space bar. (Your mouse cursor will become a camera icon.)
Take the picture of your browser window clicking on it with your mouse.
As you can see on the example below, my screen has a nice drop shadow around its minimal browser frame, with no other tabs showing or distracting favorite bars with colorful icons.
If you can’t understand the instructions above, don’t shrug and ignore it. Please let me know what part you don’t understand, so I can help you and you don’t get docked on your assignments.