Once the wort has cooled to about room temperature you can add the yeast. It can get a better start if you cool a pint of the wort in a seperate glass and then add the yeast to the pint of wort. Once it's given a start in there you can add it to the room temperature wort. In this case we waited overnight before the pint of yeast mix was added to the fermenting bin. This is also known as pitching the yeast.