Select the deck stain and sealant products of your choice. Look for stain products that are rated for longest wear, particularly in locations where the deck is exposed to a lot of sunlight for long periods every day. Cheap products are not worth the savings -- you will just have to restain your deck nearly every year if you scrimp on product now.
Clean your deck thoroughly. Use a power sprayer, with the nozzle set on "wide," to remove all dirt and any debris from between the board cracks. Don't damage the wood grain by using the power sprayer too close to the wood, or on a narrower setting. Let the clean deck thoroughly dry overnight.
Mask any areas of your house or siding that are directly adjacent to your deck. Use plastic drop cloths and masking tape to protect these surfaces from stain splatters.
Block off all approaches to your deck. Restrain any small children or pets from walking across the deck while you're working or while the stain is drying.
Mix multiple gallons of stain product into one 5 gallon bucket to ensure a consistent color. Scrape the bottom of the gallon containers to remove all colorants and chemicals that may have settled on the bottom and add this material to the 5 gallon bucket. Stir the stain thoroughly.
Prepare your staining brushes or roller applicators and trays. Place any trays or buckets on a heavy plastic tarp to avoid creating unintended rings of stain on the deck itself. If you will be using a roller or stain applicator "stick," tuck a good stain brush in your back pocket as well. As you apply stain, use this handy brush to reach any touch-ups.
Begin applying stain onto the deck with a roller applicator, working from left to right if you are right-handed, or right to left if you are left-handed. Make sure that at the end of your project you'll have an exit from the deck -- don't paint yourself into a corner.
Lay down a first "line" of stain -- working along the lengths of the deck board, not across them -- a few inches away from the edge of the deck or the side of the house. Don't use a brush to "cut in" stain around the edges of the deck first. Instead, lay down a long line of stain and then reach in with a brush to wet-blend stain between the line and against the house. "Cutting in" around the deck with a brush before staining the rest of the deck usually results in an unsightly dark band around the perimeter.
Make sure you work stain into all cracks and crevices along the boards. If you use a roller or a staining stick, press down on the applicator head so excess stain oozes into these cracks evenly. Examine the cracks between the boards from three angles: top down, far right, and far left. Apply more stain to fill in any bare spots you see.
Add more stain to your roller tray or applicator stick after you have restirred the stain in the big bucket. You don't want colorants to settle to the bottom of the bucket during the application process. Cover the 5-gallon bucket when you have stained the entire deck.
Allow the stain to dry thoroughly, overnight, before allowing traffic on the deck or pulling up the masking and putting away your leftover stain. Preserve any stain left over for future use by pouring it back into the original 1 gallon cans and sealing the lids tightly.
Apply a top coat of sealant using clean brushes or applicators if your stain product doesn't include a heavy-duty sealant. Sweep the deck clear of any debris first, using a soft bristle shop broom. Apply the sealant in a method identical to the application of the stain itself. Let it dry overnight.
Remove the masking and pour any leftover sealant back into 1 gallon containers, sealing the lids tightly. Dispose of any brushes or other applicator heads responsibly.