This is slightly harder than in say Saint's Row., but still rather easy none-the-less.
Hit R1 to enter TTD and then aim ysibg L2 to make it easier. You'll know if you've done it if they bend over standing up. If they fall down, all you've managed to do is cripple them. Aim slightly more toward the stomach.
Use a light machine gun or a Spacer's Choice basic pistol to make it easier.