Find a civilian in their house with nobody else in it, make the person your friend, lightly punch him/her, take out your shield, and wach your block stats go up. This prosses might take a while. It will also raise your heavy or light armor skill bepending on what type of shield you are using. You will still get hurt a bit depending on what blocking level is on the shield. (make sure that you don't kill the person, you might need them in another mission.) Press X when you want to stop fighting.