How do I pause in League of Legends
You can only pause in Custom Games using Tournament Draft mode using the /pause chat command. You can then unpause the game using /unpause or /resume. Here's a quote from the official competitive site:
This feature is only available in Tournament Draft Custom Games. The commands to use the pause feature are, as you might have guessed, /pause and /unpause.
You cannot pause games, unless it's a custom game.
You can pause Tournament Draft games, by simply typing /pause in chat and unpause it by typing /unpause in chat.
Good luck mate.