Presentation of the activities
Stand centrally, where everyone can see and hear you. Ensure quiet, so you don’t have to repeat yourself. You might have to make clear that you wont start until its quiet. You might need to get the kids to sit, so they don’t run everywhere else.
Talk loud and clearly. Start with the name of the exercise, so the kids can guess at what they will be doing. You could find a background story (could be totally without any reality to ensure fun), you could also compare to other known exercises.
Explain the rules of the game as simple as possible. Some rules can be left out until the game has started. Depending on your group, additional rules might be required during the exercise. When the kids has done the exercise a couple of times, you could add new rules or variations to keep it entertaining.
Ensure that the majority has understood the rules before you start. Some rules can best be explained through demonstration, so that the kids can see what to do.
Be consequent with the rules during the game, but admit if you did not see a rule violation and abstain from doing something about it.