Match stain color for the new deck boards as closely as possible to the color of the existing deck boards.
Stir the stain thoroughly and pour some into a clean bucket.
Brush one coat of stain on the new boards. Allow it to dry as recommended on the label. If the paint store has done a good job, it should only require one coat of stain for the new boards to match the existing ones. Unless you plan on staining your entire deck, you're done.
Remove all furniture and other items from your deck. Sweep it with a broom, paying attention to get all the dead leaves and debris from between the boards.
Apply a deck-cleaning solution if your deck is very discolored or mildewed. Application methods differ depending on the product, but typically you will apply the solution with a roller, brush or garden sprayer, let it work for 30 to 60 minutes, and hose it off. Allow the deck to dry completely.
Brush on one coat of deck stain, working in lengths of three or four boards at a time to avoid marks where the stain overlaps. Stain both the existing boards and the new ones. Since the new boards now match the existing color, they'll blend in perfectly with the second coat of stain.