To create bleed in your design, simply make sure the background color, graphic, or image you want it to bleed extends off the the final trim edge of the page by .125" (3mm).
If you are creating your file in an application such as Photoshop, you must make your document height and width dimensions .25" (6mm) larger than your final trim size. For example, if the finished size is 8.5" x 11" then make your document 8.75" x11.25". Position guides that are .125" from each edge. For design purposes, these guides will represent where the paper (and any of your design elements) will be cut.
Position any background color, graphic, or image that you want to bleed so they extended past your guides, all the way to the outside edge of the document. Remember, the extra .125" will be cut off after your piece is printed.