You need to stitch the pattern x 2 leaving 2-3cms (approximately 1") gap between each box

Iron on fusible interfacing inside the 4-sided stitches

Trim off the excess fabric allowing 1cm (0.39")

Press the corners inwards and trim off the outer corner

Press the outside edges inwards and mark the centre by enlarging a hole on both pieces

Lay the pieces as shown in the picture with one corner lined up with the centre of the second stitched piece 

Leaving approximately a 7.5 cms (3") tail stitch both pieces together working your way around the biscornu

Keep checking that the four sided stitches line up

Leave two sides unstitched and stuff with Polyfill - I used a chopstick to push the Polyfill well into the corners 

Close the open sides and finish by tying the two ends together and losing inside the biscornu

Thread a needle with a longish length of strong thread by doubling up to leave a loop.  Put the needle through the button back to front and then back down from front to back. Put needle through loop so that the button is locked onto the thread. Now push the needle through the middle hole on the front of the biscornu coming out through the hole on the back. Put the needle through the second button as before. Now work the needle through the biscornu to come out at one of the holes in the first button. Go back through the other hole and do this 3-4 times pulling taut. It should produce a dimple in the middle of the biscornu on both sides. To finish come out through the centre hole in the fabric and to the side of the button and wrap the thread around the button and then lose the threads inside the biscornu.   

This chart can be purchased *here* for US$8 

All pictures can be enlarged for a closeup view by clicking on them