Wash your berries as normal. I place them in a bowl and add water and a splash of vinegar. Then I quickly rinse again with clean water.
Simply hold a strawberry between your fingertips and hold the straw against the bottom of the strawberry. Line up the straw, so when you press the straw through the strawberry, it will go through the pith (the flavorless center) and come out the stem where the green is removing the entire cap of the strawberry and taking some of the pith with it.