For the top stair rail, hold a 2x4 against the top and bottom posts, parallel with the stairway. At the top, mark the rail for an angled cut. Still holding the stair rail in position, mark the lower rail post (Step 2) for an angled cut.
Use a layout square and T-bevel to mark lines around the rail post. Set a circular saw blade to a depth about 1/4-inch more than the thickness of the post, and cut both angled lines. Have a helper hold the post, gently lifting upward as the cut is finished.
Cut the bottoms of the stairway balusters at 22-1/2 degrees -- just like the deck balusters. Then hold a baluster plumb against the angled railing, and mark the top angle. Gang-cut the balusters and fasten them in place using a scrap 2x4 spacer.
Where the stairway rail cap meets the deck rail cap, hold a board in position and mark it alongside the post for a bevel cut. Adjust the angle on a circular saw and experiment on scraps until it is right. Attach the cap rail with 2-1/2-inch screws.