After the piece of fabric has been pinned to the blocking board I steam it gently with a steam iron. Basically I set my my household steam iron to a high temperature and steam setting and then hold it horizontally over the knitted fabric about 1 cm away from, it continuously moving the iron backwards and forwards over the work.
This 'kills' the yarn and relaxes it. This means that once it has cooled down and the pins are removed from the blocking board you are left with a more supple and relaxed piece of fabric which no longer roles at the edges...it can't as the springiness has been relaxed. Steaming or 'killing' the yarn also makes the fabric stable so that once the item, if it were a cardigan or sweater say, has been worn and then washed it will return to the original shape the maker intended it to be. That's not as important with a simple cushion such as this but it does make the fabric easier to work with. It is now 'square'and my seams will line up nicely.