All you have to do is tear the batt along its length until about an inch or so from the end. Then, tear the batt from the same end along its length again. Repeat until the batt is gone and you're left with something like this:
Here's a breakdown:
Note: this might not work well with a really directional fiber that's been carded into a really directional batt (all the scales on the individual fiber are facing the same way). You could get a batt like that if all the fiber was fed in one direction, say shorn end first. Directional fiber can be sort of hard to draft if you try to draft against the grain, so to speak.