Half-time basically works in exactly the same manner as any opportunity to reassess your side's tactics, with the key difference that you get to address your players directly.

At the break
In any game it's unlikely all of your players are going to perform at the same level, so you can use the drop-down Teamtalk buttons next to each player's name to issue a brief assessment of the way that player is playing.
When you issue a team talk assessment you must take into careful consideration the personality of the individual concerned. You should be able to deduce from this, and any notes you've made on the player, whether they are more likely to respond to encouragement or criticism if they are playing badly. And remember to take advantage of the stats options on offer to judge exactly who has been putting in poor performances, rather than those who haven't been fed the ball by their team-mates or those who have had the run around from vastly superior opponents.
The effects of your teamtalk will appear in the commentary, with vital clues contained in the text of the accompaniment to your match. A player suddenly performing with more energy has clearly been inspired by your words during the break. Be sure to check how the ratings at full-time compare with a players' overall performance at the break to see if your words have had a huge impact.
Of course, when choosing words to berate or encourage your stars you must also be conscious that they'll still feel the effects of your dressing downs after the game. Expect news items to appear commenting on the happiness or disillusionment of players who feel you have treated them well or badly during your team talks. While there is a chance you will anger a player with harsh words during a game, you should also note that this invaluable opportunity to speak with them directly affords you the perfect chance to turn around a relationship that has begun to sour.

Post Match
At the end of the game you'll also get a chance to address your players on an individual basis or as a group with the same drop-down options. These assessments of your player's contribution to the game are all the more stinging when they are negative because the player involved does not have the opportunity to respond on the field. Branding an individual an embarrassment is a risky policy that could lead to negative Press and may cause a serious and instant deterioration in your relationship with your player.

Say Nothing
Sometimes it's best to say nothing at all. Even Football Managers sometimes get carried away in the heat of the moment and berate players when a dramatic silence would speak volumes. Many of the players you'll manage will be all too aware of their performance level you and what the fans expect of them, and the more professional ones will berate themselves if they don't get it right. If you are not sure how a player will react, or if you have not properly assessed the player's contribution to the game, you'd be well advised to adopt the 'silence is golden' approach.
Be wary of ignoring team-talks for long though - players need instruction and they will become uneasy at a lack of direction from their manager. If you don't wish to conduct team-talks yourself, you can leave it to your assistant or coaching staff by ticking the 'Assistant manager/coach takes charge of team-talks' in manager options.