Lay out the Douglas fir decking on an open work area, upside down. Brush weatherizing primer over the back and sides of each board. Let it set for 24 hours.
Measure the length of the porch surface, in the direction perpendicular to the joists, so you're crossing all of them.
Cut a decking board to that length, using your miter saw.
Set the cut board along one side of the deck, with tongue side facing in toward the deck. If there's a wall there, leave a half-inch space between the grooved side of the board and the wall. Secure the board in place with two nails from your nail gun through the face of the board at each place it crosses a joist.
Cut the second plank to size. Set it next to the first one. Set a piece of scrap decking at the outer edge of the board and tap the scrap wood with your hammer to get it tight to knock the board tightly against the first one. Shoot nails through the side of the board, just above the tongue, at an inward angle, at each point where it crosses a joist.
Repeat the process to cut and install the rest of the boards on the porch. Length-cut the final board on a table saw, if necessary, so it fits alongside the end.
Brush a layer of weatherizing primer over the tops of the boards. Let it dry overnight. Paint the surface of the porch.