How??

When the enemies soul gauge is flashing red, break some of their armor and when a big red explosion happens press RB.

When a big red explosion happens press RB.

Once the soul guage is flashing read, continue to attack, if they keep blocking your attacks eventually they will flash and be stunned for a short while. This is called a soul crush. To activate a critical finish, you must press A+B+K+G (or whatever you assign on your pad to represent this) during this stun period. This means that if you attack with a combo and the first hit causes the soul crush, chances are you'll be too late to do the critical finish by the time your combo ends. So try to anticipate how many hits it will take to achieve the soul crush once the meter is red and don't use long combos if you think it's about to happen.

The easiest character I find for doing this against the computer opponents is Yun-Seong. If you continually use forward+B,B the computer tends to just keep blocking it (unless they are very easy opponents who keep getting hit instead). Pretty quickly you'll get a soul crush and an easy critical finish. As noted above, get to know when the soul crush is about to happen and don't press the second B if you think one more hit will do it. The easiest place to do this is in Tower of Souls, floor 11. They block a lot on this stage and you will get very accustomed to seeing his critical finish animation!

Rob

Soul Crush is when, after after, continually attacking a guarding opponent or breaking their armor (or both), they flash with red lightning. The red lightning part is Soul Crush. Critical finish is what everybody else has been saying about pressing RB when the red flash happens.
A common misconception is the belief that all armor must be broken. It doesn't have to be, but breaking armor helps speed things along, and will probably happen along the way to a Critical Finish, anyway.
Of course, I now reread and saw that Rob said the same thing, basically. I, myself, figured this out after reading m instruction booklet, which came with my game disc when I bought it. Seems people have forgotten lately what those things are for...