Write an article based on this "Determine a length for the head casing. Cut the head casing. Use a miter saw to cut sloped casing. Nail down the head casing."
The simplest way to find out exactly how long your top piece needs to be is to subtract 2” (5cm) from the sill length. That way, you can ensure that the rest of the trim will be proportional to the sill. A window that’s 28" (71cm) wide might therefore have a head casing that’s between 30-33” long.  It’s common for the upper casing to be about an inch (roughly 2.5cm) shorter than the sill on either side. If you’re using the same casing for the top and sides of the window, the distance that the top piece extends on either side should be equal to the width of the trim. Draw a line with a pencil to indicate the appropriate length of the top piece. Make a perpendicular cut straight across the casing. Mark the length of the top piece from the end of the casing to save yourself an unnecessary cut. For windows with a uniform trim on the top and sides, you’ll need to cut the edges of the top piece at a 45 degree angle. This can be accomplished using a miter saw. Cut the casing one one side, then turn it over and make a second cut in the opposite direction.  Before you cut your sloped casing, mark your window measurements on your trim piece. Then, cut your 45 degree angle moving away from the measurement lines on both sides of your trim. The shorter part of your head casing piece needs to match your window measurement. The edges of the finished top piece should now fit together perfectly with those of the side casing. If you don’t have access to a miter saw, you can also use a miter box and a normal handsaw to achieve a precise angle. Rather than aligning the top piece flush with the upper edge of the window opening, raise it up by ⅛” (.32cm) to leave a subtle reveal line. Fasten the head casing using finishing nails, remembering to space them out every few inches across the length of the board.  Use a level to confirm that the top piece is even before nailing it down.  The reveal line will lend the appearance of added depth to the finished window. If your side and head cases look a little misaligned, you can fix this with a long finish nail. Drive the long finish nail into the top of the side casing and into the head casing, which should straighten them.