Find and fix the moisture source
Open windows and run fans to dry the area
Use a wet/dry vacuum to remove standing moisture
Sprinkle baking soda over the carpet
Let the baking soda sit for several hours
Vacuum the baking soda thoroughly
Spray a mixture of white vinegar and water lightly on the affected area
Blot the carpet with clean towels
Apply an enzyme cleaner designed for odors
Repeat cleaning if the smell remains
Use a dehumidifier to keep the room dry
Steam clean the carpet if needed
Replace carpet padding if it has mildew damage
Call a professional carpet cleaner for severe odor
Remove and replace the carpet if mildew is extensive