Test the pressure-treated wood with a moisture meter. The reading should be 20% or less in order to achieve optimal results. If the moisture content is higher, wait a few weeks and test it again until the moisture content drops. The stain will not adhere well if the wood contains too much moisture.
Wash the posts with deck cleaner or use a garden hose to spray away light dirt and debris. Wait for the posts to dry.
Lightly sand the wooden posts. Sanding opens up the wood's pores and helps it to absorb the stain evenly. Brush or vacuum away the particles from the surface of the wood.
Put on gloves and apply the cedar stain according to the manufacturer's directions. Use a brush to apply the stain to get into all the crevices and the undersides of the posts. Sprays provide faster coverage but are messier.
Reseal the deck and posts yearly to protect the wood.