Designing in Photoshop - How to add a bleed.

If you are designing something to be sent off for professional print, it is essential you add a "Bleed" A bleed is an area of  "No mans land" around the design that will be cut off during printing, if you fail to add a bleed some of your design will be cut off.

To add a bleed go to Image > Canvas size

Then you'll see the canvas size window that looks like this, you'll want to add about 6 or 7 pixels. I added 2mm

If you do this after you've finished designing you wont have to worry about it while you're making it. Remember to select and delete anything that is in the bleed area, though. 

As you can see I've done here.