• While in a Competitive Game hold tab
  
• Use your arrow keys to navigate to the player's name
  
• Press enter and enter again on the "Commend" selection
  
• Select one of the three choices to commend

A brief definition of what each one means so you can commend accordingly:

Friendly: The player has been nothing but good to you. He never once swore or made fun of you the whole game but instead tried to help you and encourage you.

Teacher: The player has taught you something new that helped you in your overall gameplay and tells you tips for future reference.

Leader: The player has been calling strategies for each round and tells you positioning to secure the sites and take the wwinning rounds.