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Use a pencil to draw a small ‘X’ where the jack will go. Don’t obsess over the placement too much—you’ll be able to make any necessary adjustments when marking the exact location of the mounting bracket. Make your mark dark enough to be plainly visible so you won't have to hunt for it while you're doing other things. Once you’ve got the bracket situated right where you want it, run the tip of your pencil along the inner edge of the wall plate mounting bracket for your network jack. When you’re done, you’ll have a rough outline that you can use as a template for cutting the hole for the wall plate.  Line the wall mounting bracket up as evenly as you can with the nearby electrical outlet. If desired, use a bubble level to ensure that the top and bottom edges of your outline are perfectly parallel with the floor. Head to the main circuit breaker panel or electrical control box for your building and flip the switch corresponding to the outlet you’ve chosen to install your network jack next to. Doing so will minimize your risk of electrical shock, as you’ll be working close by.  Individual circuits should be plainly labelled on your breaker panel. If you’re attempting to install an Ethernet jack somewhere other than your own home, you may need some assistance finding the right breaker. If for some reason your breakers aren’t labelled, or they’ve been labelled incorrectly by mistake, an electronic circuit breaker finder can help you figure out which breaker to flip.

Summary:
Mark the spot on the wall where you want to place the jack. Place your mounting bracket over the marking and trace around the inside. Shut off the electricity to any electrical circuits in the immediate vicinity.