Installing The Window Stool

• 1). Place a straightedge horizontally across the window to make sure the window jamb is flush with the wall.
  
  
• 2). Hold a side casing vertically against the wall, making sure it is aligned with the right side window jamb. Make a reference mark on the outside of the trim with a pencil. Repeat for the left side jamb. This is done to determine placement of the window stool.
  
  
• 3). Measure the distance between the pencil marks with a measuring tape, then add 2 inches to the measurement you get. Cut the window stool to this length using a power miter saw set to 7 degrees.
  
  
• 4). Hold the window stool face down against the windowsill and use a pencil compass to mark where it meets the window jamb. Cut the window stool along the lines you made using a jigsaw.
  
  
• 5). Place the window stool face up against the windowsill and use a pencil compass to mark where the stool's end touches the window jamb. Cut the window stool along the lines you made using a jigsaw.
  
  
• 6). Nail the window stool to the windowsill using four 4d (18-gauge) finishing nails and a hammer.
  
  

Installing The Window Casings

• 1). Stand a side window casing on the right side of the window stool and mark with a pencil where its inside edge meets the top of the window jamb. Cut the casing along that line using a miter saw set to 45 degrees. Repeat for the left side of the window stool.
  
  
• 2). Align a side window casing on the right side of the window and nail it to the window jamb using four 4d finishing nails and a hammer. Repeat on the left side of the window.
  
  
• 3). Place the top window casing upside down on top of the side casings and mark with a pencil where it meets the inside edge of each side casing. Cut the top casing along those lines using a miter saw set to 45 degrees.
  
  
• 4). Apply wood glue to the top casing and glue it to the top window jamb. Hammer four 4d finishing nails into the top casing.
  
  
• 5). Hammer 8d finishing nails into the outer edges of the top and side casings to secure them to the wall.
  
  

Installing The Window Apron

• 1). Place a piece of window casing face down on the window stool and mark with a pencil where it meets the outside edge of each side casing. Cut the window casing along those lines using a miter saw set to 45 degrees to create a window apron.
  
  
• 2). Hold a piece of window casing perpendicular to the window apron and trace the profile of the apron onto the window casing with a pencil. Cut the window casing along the line using a miter saw set to 45 degrees to create a return (mitered ending). Repeat for the other side of the apron.
  
  
• 3). Glue the returns to the ends of the window apron with wood glue.
  
  
• 4). Position the window apron on the wall beneath the window stool, then nail the window apron to the wall using four 8d finishing nails and a hammer.