You can make your own zombie outfit using clothes you don't need or don't like anymore. This is especially true for shirts, which rip and tear without using much force from scissors or a knife. Cutting your own zombie shirt requires getting creatively random, rather than strategic, about where you place the rips. Zombies don't do anything with rhyme or reason, so your shirt should reflect this carelessly deranged attitude. You can add dirt smudges or blood before or after you cut the shirt, but if you don't want these particles on your cutting instrument, leave this step until afterward.
Lay the shirt out on a counter or other flat surface. Do not cut it while you're wearing it.
Envision how the shirt would rip if you were clawing it with zombie fingernails while wearing it. Most of the rips would be located near the upper chest and across the stomach, in horizontal and diagonal directions.
Use the scissors or shears to start a rip in the shirt. Put the scissors down, and continue ripping from this starting point with your hands. This will make the shirt fibres fray, rather than appear sliced in straight lines.
Gather small portions of the shirt in a handful. Snip chunks of material out of these handfuls with the scissors. This will create patches of jagged material.
Cut and rip the shirt at the seams. Zombiephiles.com recommends ripping at the shoulder seams especially, as this replicates the struggle you would have gone through when vainly trying to escape a zombie attack as a human.
Rub the shirt with sandpaper to create faded, worn areas that you can't duplicate with a pair of scissors.