Staining your composite deck not only protects it from the elements, it can also change the color of the composite's surface to make it look more attractive. Before you stain your deck, check the weather. Try to stain it on a day when there is little to no chance of rain. Also, make sure you start early enough that you will have time to check the dry finish before it gets dark.
Move your deck furniture out of the way, preferably off of the deck altogether. Sweep the deck and spray down the boards with a garden hose. Let the deck dry overnight. Put plastic sheeting over any plants that could potentially get dripped on.
Open the can of finish and stir the liquid with a wooden dowel. Put the head of your finish brush into the can. Wipe the brush on the can's rim to make sure it's not oversaturated. Walk to the far left corner of the deck and begin by putting an 1/8 inch thick coat of stain on each board, following the grain of the wood. Keep your strokes even and smooth so the stain doesn't get lumpy when it dries.
Continue staining until you finish the rest of the railing, including the top and sides. Don't miss any slats. Once you're done, go back to that far left corner and start on the deck's boards using the same technique.
Put the lid back on the can and wait half an hour for the majority of the stain to dry. Check to see if there are any spots with drips or pools. Wipe your brush on a cotton cloth. Then, brush on top of these spots as if you were applying more stain, only this time your brush will pick up the stain instead of applying it.
Check your deck after another half hour. Sand off any bumpy spots with fine sandpaper. Repeat Steps 2, 3 and 4 to apply a second coat. Sand off any rough spots and touch up with more stain as needed.