Pour the grape juice and fruit punch concentrate into your punch bowl or a large pitcher. Stir everything together well, making sure the concentrate has fully dissolved into the juice with no clumps left behind.
If you have the time, cover the juice base and refrigerate it for about 30 minutes before your party starts. This step deepens the flavor and means you'll need less ice later, which keeps every glass tasting bold instead of watered down.
Just before your guests arrive, pour in the chilled lemon-lime soda and give the punch a gentle stir. Adding it last keeps all those bubbles intact instead of letting them fizzle out in the fridge.
Drop in a handful of ice cubes, or better yet, frozen blackberries or grapes, to keep everything cold. Frozen fruit is my favorite trick since it chills the punch without diluting the flavor as it melts.
Float orange slices and fresh blackberries on top for that classic spooky look, or slide an orange slice onto the rim of each glass. Serve immediately while the punch is still bubbly and ice cold.