It is important to moisten your canvas before stretching it. Wring excess moisture out of a wet towel and gently wrap it around your work. Pat the towel gently. The moisture loosens the stiffner in the canvas which can now be stretched and pinned into shape and allowed to dry naturally.

Lay a sheet of white paper on a wooden board. Using a pencil draw an outline of your finished work on the paper marking the centre points on each side. Now lay your moistened carpet right side up onto the paper and match the centre points of the carpet with the points on the paper. Starting from the top centre and working outwards to right and left, insert brass drawing pins outside the worked area at intervals of about 2.5cm.  Pull the canvas straight and pin along bottom edge to match drawn shape.
Match, pull and pin the sides in the same way and then leave the canvas to dry naturally.  This may take two to three days.  If the canvas is still not quite square when unpinned, repeat the whole process.