Final answer:
To start a WBC differential count, it is best to select the monocellular layer on the blood smear between the thickest area and the feathered edge.
Step-by-step explanation:
When performing a WBC differential count, it is best to start by selecting the monocellular layer on the blood smear. This layer is a single layer of cells and is located between the thickest area and the feathered edge of the smear. By selecting this layer, you can ensure that you are counting a representative sample of the different types of white blood cells.