A picture being worth a thousand words makes screenshots often the most convenient method for conveying instructions to people. While Apple has a number of screenshot options built into OS X, these often result in intermediary files that can clutter up your computer if you frequently use them.
For example, if you press the classic Shift-Command-3 shortcut to take a screenshot, the image file will be saved to your desktop, where you can open it, crop, and otherwise edit it before sending it to your recipient. With this method, though, you are left with the file on your desktop that you will have to throw out.
If you want to avoid this inconvenience, you can take screenshots and mark them up for sending to others without creating a single intermediary file. To do this, first take a screenshot using one of the following methods:
- Shift-Control-Command-4 followed by pressing the spacebar
The first of these will create an image of the entire screen, the second will allow you to specify a selection of the screen, and the third will allow you to specify a window to take a screenshot of. The key here is the inclusion of the Control key in these shortcuts, which saves the screenshot to the clipboard instead of as a file to your hard drive.
With the image in the clipboard, open Preview (you can use the Spotlight menu with the Command-spacebar shortcut to quickly do this), and then press Command-N to create a new image from the clipboard in Preview. You should now have an image available, and can use Preview's toolbar to crop, add text, shapes, and other markups as you see fit.
When finished editing the image in Preview, you can press Command-A to select the entire image, followed by pressing Command-C to copy it as-is, including all of your edits.
Now you can open Apple's Mail program and paste the copied image into your message, sending it off without having to manage a single file on your hard drive.