SAFETY TIP: When training fielders, many coaches will be sorely tempted to keep two or more baseballs in play at the same time on the practice field; (one going to the infielders and one to the outfielders, for example.) It speeds things up and makes for less boredom and waiting around. DO NOT DO THIS UNDER ANY CIRCUMSTANCES. Baseball is a pretty safe sport, largely because it is easy for most players to see a baseball that is about to hit them, and either catch it or get out of the way. When two or more baseballs are flying around, this becomes much more difficult. It becomes much more likely that a player will get hit in the back of the head (concussion), or full in the face (broken nose, broken teeth,) with a thrown or batted baseball.

"Even a fool can learn from his own mistakes. A wise man learns from the mistakes of others" - Chinese proverb.

In general, young fielders will immediately want to show off by fielding baseballs the way the have seen it done on television. Incredibly, many athletic, well-meaning parents actually encourage this. Don't do it. The fact is that young ballplayers are nowhere close to having the skills needed to catch a fly ball one handed (or back-handed,) RELIABLY. Sure, they can do it once in a while in your back yard or in practice, but to win baseball games, the fielders must be able to catch those fly balls and stop those hot grounders nearly 100% of the time.

To do this, the manager will teach your young ballplayer to catch even the easiest pop flies by:
First: hustling to get right under the ball, and
Second: Using both hands to catch the ball, the bare hand backing up the gloved hand. This may not look particularly spectacular, but it works reliably. In a close game, it sure looks a lot better than a dropped ball.

SAFETY TIP #2: When a fly ball is hit to the outfield, two or more players may go after it at the same time, and collide. To avoid this, train your fielders to wait until the ball has peaked, then call for the ball:
"I've got it!"
and to respond: "Take it, Billy!"
and to do this THREE TIMES.
This good habit reduces fielding errors, wins games, and prevents injuries.

Fielding a ground ball takes a similarly conservative approach. The manager will teach your young fielder to get square in front of the oncoming ball; then get down on the left knee (for a right-handed fielder) forming a "V" with his left shin and his right foot; this traps the ball; put the glove in the center of the "V" to catch the ball; then use the bare hand protect his face ( and catch the ball ) in case the grounder takes a bad hop.

Train your baseball players to watch the ball go all the way into the glove, and to keep on watching it until they are holding the ball firmly in both hands.

Make sure that your baseball players do not look away prematurely, regardless of how hectic things get during the game. This is important. It prevents bobbled balls and fielding error. It also prevents injuries.

This is not at all the way they do it in the major leagues.
It's a different game.