Mitered corners will give your cushion covers square corners with neat vertical seams. Whether you've sewn a cover yourself or bought a premade cover without square corners, you'll first need to turn it inside out. Make sure all four of the corners are pulled all the way out, including the little points. Measure the cushion's thickness. Measure and mark a line that's as long as your cushion's thickness diagonally across each corner. Sew along these lines. Now, when you turn the cushion cover right side out, you will have square corners.
A box cushion is shaped like---you guessed it ---a box. It has square corners, but they're softer and less sharp than mitered corners. Rather than simply being made of two rectangular fabric pieces sewn together, a box cushion has a gusset, or long strip of connecting fabric that's sandwiched in between the two rectangular pieces. When all the pieces are sewn together, they create a boxlike shape with square corners. Box cushions often include piping (also known as welting), which helps the cushion keep its shape. When you sew each rectangle to the gusset, the piping goes in between the two pieces. As you sew round what will be the corner, bend the piping so that it makes a slightly rounded corner. It's hard to make sharp square corners on a box cushion; the piping won't bend into such a severe angle.
The simplest way to make square corners on cushion covers is by sewing a knife-edge cover. This method won't work on a three-piece box-style cushion: it's best to use a pillow-style cushion, which consists of two rectangular fabric pieces sewn together and stuffed, without a gusset connecting them. The first step to creating sharp corners is clipping your seam allowance at each corner. Make a diagonal cut across each corner, trimming off excess fabric without cutting through your seam. Next, turn your cover right side out and iron the corners flat. That will help them regain their square shape. With a pin, gently grab the fabric at each corner and pull it out so that there's no fabric folded under the seam. Iron the corners again. Now they should be sharp squares.