Use a fork to pierce through the top of the strawberry just a bit below the base of the stem. If your strawberry has any white at the top you can pierce it where the white meets the red. Hold the strawberry and turn the fork on the horizontal axis (like you're twisting the strawberry and fork in opposite directions) to easily remove the stem. If you twist and some of the stem remains, then you'll want to pierce the strawberry a little lower down the next time. Once you get it, you'll be able to get it every time after that! You might not remove the center (core) of the strawberry every time and that's okay! The more strawberry the merrier!
This is a great method to save you time for de-stemming lots of strawberries, then dice them and freeze some in a freezer-safe mason jar!