I always think of it like this: first look at the where the pass goes, then look at the setter, after the setter has set the ball look for the hitter. Try to align to front of your hands with their hitting hand ( if you're blocking alone). If you're blocking with multiple people (for example 2) try to keep his hitting arm in between you and the person your blocking with. If the hitter is in a very awkward position to hit they will usually go for a tip, roll shot of a block out.