Alright, here is how you go about it.
Firstly - Judge the length. Clearly the most important. Only time one should defend a pacer on the front foot is when the ball is, obviously, full/good length. Doing it to a bouncer or a yorker is just foolish. Once you are assured that the length is correct, move onto step two..
2) The Line. It is very important you get your body right behind the ball, avoiding to play away from your body or getting cramped for room. Also avoiding playing those you can leave safely. Then there is step 3...
3) Technique. Most important. Now, there are some key points to remember for this:
- Foot to the pitch of the ball (Or as close as possible)
- Keep your elbow high and straight
- Your head should be right in line bat at all times, staring at the sponsor's sticker on the back of the bat.
- Even grip with the top and bottom hand equally tight on the middle of the handle
After you've got your technique sorted, make sure to watch out for the following:
-Swing, early or late.
-Cut
-Seam movement
As this is hard to predict, it is best to leave playing your shot till as late as possible. In order to do this you need to have quick feet and a solid technique so that you waste as less time as possible getting into position.