Content Previews Tutorial
Open up your canvas, any size you want it to be. Your layer will have a padlock next to it. Double click it. On the box that pops up, just click 'OK'.
Duplicate your background layer. Move the duplicated layer to the very top of the layers panel.
Fill your duplicated layer with any color, set the Fill to 0%.
Go to blending options and create a stroke black at 1px. Click 'OK'.
Merge that layer down. Duplicate your layer. Go to blending options and make the stroke 2px white. Click 'OK'.
Mer that layer down. Duplicate your layer once more. Go to blending options and make the stroke 3px black. Click 'OK'.
You'll put your content in a layer below the stroked layer, so it should be in the middle layer. Make sure you save a PSD format of your preview so you can quickly open it for later use.