Taking a screenshot is common knowledge. However, when you're attempting to take one of a browser window or an email, it might require some cropping and fine-tuning afterwards to trim the background edges. This can go the other way around and maybe you didn't capture the entirety of what you wanted to include. These simple extra steps in taking screenshots allows you to choose exactly how much or how little of a window or screen you want rather than manually moving the cursor or worrying about preciseness.
How to take individual window screenshots
Press Shift + Command + 4 + Spacebar. The pointer will change to a camera icon
Click the window or menu to capture it. To exclude the window's shadow from the screenshot, press and hold the Option key while you click.
More info + additional screenshot options: https://support.apple.com/en-gb/HT201361