I'm afraid there's no simple answer to this question. Narrowing ranges both pre and post flop is, without doubt, the most fundamental and at the same time trickiest part of poker play. Poker is NOT (only and fundamentally) about probability, it's about narrowing the range of your opponent(s). And that is achieved by observing consistency into action, in the current hand and in previous hands.
The best way to TRAIN this is to go through hands and hands excersises where you simply observe the hands being dealt and the action on the table and, before showdown, you try to put everyone in the action on a hand range (not a hand, a hand range), and then record how many times you are correct. For each hand, write down HOW you came to the range. WRITE IT DOWN, don't think it on your mind, literally write it down before you see the hand. Then, after you see the hand, revisit your logic and see how the information modifies your perception. Was the hand on the range you assigned? Did you made an ungranted assumption? Does seeing the final hand change your logical deductions? How? Did you miss anything?
Repeat this enough times and you'll start developing a knack for reading hands. Training on reading people is a bit more difficult as you don't get so many samples. The best bet is to review your own tournament play focusing on opponents and not cards. Try to identify players as loose, tight, fish, random and try to identify how they play based on the information of the hands you can see and how they act. This is a bit trickier becuase usually you need a HUGE database to be able to follow a player through several tournaments to actually verify your judgements but even without it the actual effort of categorizing will help.
The general answer on how to do it is by gathering the information that is before you. The longer the hand plays out the more information you have. Applying that information to determine your opponent range is what distinguishes good players from bad players.
When the hand start, before a player bets, you have literally 0 information, their range is 100% of possible hands. Then action starts pre-flop, limps, raises, 3bets, calls, etc. At this point you have that information together with your read on the players over the many many hands already played. It's not the same a raise from a loose cannon than a raise from a super tight player. Of course this goes into levels, a super tight player may be aware of the image he has on the table and use that to shoot some loose bets.
After the flop the situation is exactly the same as preflop but with additional (more complex) information. Now the amount of information seem to be higher but it is actually the same as before, it is only when the players take action than the information solidifies (that's why is so important being last to act). Once the players take action, good players will take "consistent" action while bad players will take inconsistent action. Assuming you are facing a good player, their actions will hint at:
- The hand they hold
- The hand they are representing
You can't really know which of the two, you can only:
- Figure out what range of hands it is
- Whether the villain has it or pretends to have it
In order to narrow the range you have to start on your preflop range and evaluate how the players act based on the new information (the cards on the board). You must also know that how YOU act will give information out for other to narrow YOUR range, and in turn that will change how they act next.
Good players work with other people ranges and their hand. Great players work mainly with other people ranges and then use that either with the hand they have or the hand they pretend they have without much difference. That's how Obrestad won a tournament without seeing her whole cards.