General Procedure
Purify template plasmid DNA from a dam+ Escherichia coli strain (to ensure that all GATC sites are methylated for later digestion with DpnI).
Design forward and reverse primers that will bind to the region of DNA you want to mutate but that contain the modifications you wish to make. See the CAD tool PrimerX.
Run a primer-extension reaction with a proof-reading, non-displacing polymerase such as Pfu DNA polymerase. This results in nicked circular strands of the plasmid.
Cut up the template DNA with DpnI.
Transform the circular nicked DNA into a highly competent strain such as XL1-Blue. These cells will repair the nicks and not restrict the unmodified product DNA.
Select colonies with the correct DNA.