The procedure for a simple 2-step serial dilution is described in the steps below. 

1. Add 25 microliters of PBS diluent into the first well using a micropipette.
2. Transfer 25 microliters of the test solution into the first well using a micropipette
3. Mix the context by pipetting the liquid up and down twice
4. Keep the series of 2 fold-dilutions until the second last well of the microwell plate. The last well is used as a control well.

These same steps can be followed for other experiments using different volumes, and diluents.