Put on a sun hat and spread sunblock on exposed areas of skin. It's easy to get a sunburn while working outside.
Lay down dropcloths over anything you want to protect. You can leave dirt and lawn uncovered, but concrete and personal possessions need coverage.
Scrape loose paint off the deck using your paint scraper. Scrape in long, even strokes. You don't have to remove all the paint -- just the paint that's easy to remove.
Sand smooth any borders between remaining paint and raw wood.
Pour paint to fill your tray 1/4 to 1/3 full. Dip in your roller and use the textured part of the tray to spread paint evenly.
Paint the deck in even strokes, running parallel to the length of the deck boards. Allow to dry.
Apply a second, and third, coat as necessary. Allow to dry in between coats.