There are a few techniques. First, you can cut off the stem end and use a paring knife to carefully remove the core and expand the opening if needed. There are also special (affiliate link) coring tools designed especially for strawberries. You can also use a sturdy straw as your coring device. Remove the leaves and insert the straw into the core. It leaves a nice round opening that will need to be enlarged for stuffing. And I haven't tested this, but have heard you can grab the stem end, twist, and pull to remove the leaves and core.