I always make a starter. 1/2 cup DME with 2 Cups water, boil for 5 minutes, cool to the same temp as the liquid yeast pack (or close to it), pitch yeast and place a piece of sanitized aluminum foil over the top. I don't use an airlock. If you are not using a stir plate, you should shake it every couple of hours.
NOTE: You may or may not see any kind of Krausen on your starter, this doesn't mean it isn't working.
Brewers make wort, Yeast make beer!!!!