We’re now finished with our guides, so you can press Command-H (PC: Ctrl-H) to hide them because the tricky part is coming up and we don’t want anything obstructing our view.
Press the M key to call up the Rectangular Marquee tool and drag a selection around the approximate middle of our new pattern, don’t worry if it’s not exact because we’ll modify it in the next step. As always, the holding down the Shift key will constrain the selection