Make the Rulers Visible?

Just press Command-R (PC: Ctrl-R) and rulers appear across the top and left side of your image window. That’s it.

ADVANCED TIP: CHANGE STARTING MEASUREMENT

If, for some reason, you don’t want the top-left corner of the image to be measured starting at the 0x0" point, you can click-and-hold directly in the top-left corner of the image window (not in the image, in the window—right where the vertical and horizontal rulers meet), and drag the starting point to a new location (for example, if you want to start measuring 2" into your image, you could drag the axis of those two rulers to that point and it starts measuring from there). Again, this is an advanced tip for you advanced folks who are advanced, and since you’re advanced, all of that made sense. Hope that advances your advanced Photoshop skills.