A fun method of harvesting or collecting castings is what I like to call the Lighted Pile Method. It's really easy to do and a lot of fun, but you'll need some space to do this.

Lay a large heavy plastic sheet on the floor.

Dump the contents of the worm bin on the plastic sheet and divide it into 8-10 piles.

Shine a flashlight on the top of each pile. Since worms are sensitive to light, they will go down into the pile to avoid the light. You will need to remove a little bit of the castings at a time until all you have is just a pile of worms.

This may take a while depending on how bright your flashlight is. Have a friend hold a light on the pile during the entire process. Notice how the worms crawl away from the light down into the pile. Why?

Continue to do this until you are left with just a pile of worms.