First, check the existing porch alignment to ensure that your porch frame is in square. Measure from opposite end to opposite end. If the two measurements are within a half inch, you're good to proceed. Ensure that your joists are installed with the crown on the upside Make sure that your porch from pitch or slope moves down from your house. Rule of thumb: 1 inch every 6 feet.
Install z-bar flashing between porch frame and house (Image 1), allowing moisture to run away from house. Add 2x4 blocking for the porch floor to rest upon (Image 2). Add a ledger strip next to the house to allow the porch floor to be nailed to.
If you use tongue and groove porch decking, ensure that all sides are primed before you install (Image 1) in order to prevent moisture damage. The decking boards are installed to run perpendicular to the house (Image 2). Cut the boards, allowing at least a 3-inch overhang. This will be trimmed off after the porch decking is finally installed. Use a flooring nailer to install the porch deck board (Images 3 and 4). Make sure the tongue and groove boards fit tightly before nailing them into place.
After you've nailed all boards into place, come back and run a straight line across the excess out boards. Make sure and allow for any trim moldings that may go under the front of the porch. Using a jigsaw, cut off the excess wood. This will provide a straight edge for finishing (Image 1). Use a router to round the edges on the top and the bottom (Images 2 and 3). Sand the edges smooth, then prime and paint (Image 4). Always remember to prime raw wood before painting the finish coat.