After I have finished quilting the front of my cushion, I usually finish off by stitching around the entire outside edge of the cushion front. I use a long running stitch and, I find that this holds the fabric nicely in place for trimming.
Next, I trim off all the excess batting and any threads to give me nice clean edges to work with. I find using a square ruler (mine is 9.5 inches square) an advantage so that you can make sure both edges are straight and at right angles to each other.
Once the front is completely trimmed and ready, I put it right side up and measure it across the centre in both directions (top to bottom and side to side). These measurements are then used to determine the size of the pieces of backing fabric.
To work out the size of the fabric pieces needed for the back, you take your cushion front measurement from top to bottom, halve it and add 3.5 inches. You then use the resulting figure as the measurement for the height of fabric to cut x the width of your cushion.
For example: My cushion was 15 inches square. So, the top to bottom measurement was 15 inches. I halved this number giving me 7.5 inches and then added 3.5 inches, giving me a total of 11 inches. I then cut two cushion back pieces each measuring 11 inches (height) x 15 inches (width).
Once cut, you need to hem one of the long edges on each piece of cushion back fabric. However, it is important to make sure that you hem the correct edge so that any pattern on the cushion back fabric is facing in the same direction on both top and bottom pieces of the back.
To determine this, I match one of the pieces of backing fabric to the top raw edge of the cushion front (right sides together) and then lay the other piece of backing fabric on top, this time matching the bottom raw edge with the cushion front. In the middle, where they overlap, I fold back the fabric to check that the pattern is facing in the right direction on both pieces.
Pick up the backing fabric which is covering the bottom half of the cushion front and turn and press a quarter inch hem to the wrong side of the fabric along the edge which was positioned across the centre of the cushion front. Turn and press another quarter inch, so that the raw edge is hidden and then stitch in place along the edge of the hem.
Repeat this for the top half of the back fabric. Note that the bottom piece of backing fabric should be hemmed along its top edge and the top piece of backing fabric should be hemmed along its bottom edge. Remember to overstitch at each end of the hem. Trim off the thread ends.
Now match the top backing piece right sides together with the top of the cushion front, making sure that the raw edges at the top and sides match. This will mean that the hemmed edge lies across the centre of the cushion. Pin in place. Stitch the top half of the back in place. Overstitching at both ends and trimming off thread ends.
Repeat matching the bottom backing piece with the bottom of the cushion front. The hemmed edges of the backing fabrics should overlap in the middle by about three inches.
When stitching the backing pieces in place, I usually finish each corner by ending and overstitching one edge and starting and overstitching the next edge rather than putting my needle down and turning as, I find this gives a neater finish on the right side of the cushion.
To finish off the inside of the cushion before turning it to the right side, I neaten the edges with a zig-zag stitch all the way round the outside edge.
Once this is done all that is left to do is to turn the cover the right way out, paying attention to make sure the corners have been pushed out (I use a crochet hook to help). Gently give the seams a press from the right side and pop your cushion filler inside.