Step #1: You get a bruise because you are a klutz...
Step #2: Lightly cover the area with a very dry, red pigmented creme...
Step #3: Alternate thin layers of a dry concealor and powder until the bruise is covered...
AND... just for fun, can you tell which bruise is real and which is the one I recreated with my kit?! Making a bruise is simple and shouldn't take too much time or it will end up looking fake. I used the Ben Nye F/X Wheel: EW4 Master Bruise and set it with a very light amount of powder. I think a key part of creating bruises is just starting with a light application (i.e. a wash of color) and then building layer upon layer with different tints of color to give it demension. It's also necessary to keep in mind the duration of the bruise and the impact. A fresh bruise will look different than one that is a few days old. Just as a bruise from a baseball bal will look different than one from running into a wall...
P.S. The bruise on the bottom is the real one and the bruise on top is the one I recreated.