My mother made a number of these.
Use 5mm Crochet Hook (UK No. 6; US No. H/8)
As a rough guide - if your cushion is about 15in (38 cm ) square allow approx 200g of 8 Ply (Double Knitting) yarn made up of three different coloured yarns:- (Colour One: approx 60g; Colour Two: approx 40g; Colour Three: approx 100g
1st round: 3ch, 2tr into ring, (1ch, 3 tr) 3 times. 1ch, join with ss to top of 3ch. Fasten off.
2nd round: Using Colour Two, (3ch, 2tr. 1ch, 3tr) all into the same space, *1ch, (3tr, 1ch, 3tr) all into next space, rep from* twice more, join with ss to top of 3 ch. Fasten off.
3rd round: Using Colour Three, starting from the 1 ch space that forms one side of square on previous round make 3ch, 2tr, into same space, 1ch, (3 tr, 1ch, 3tr) into corner space, *1ch, 3tr, 1ch into next space, (3tr, 1ch, 3tr) into corner space, rep from * twice more, 1ch, join with ss to top of 3 ch. Fasten off.
4th round: Continuing in Colour Three (as in 3rd round) work (3tr, 1ch, 3tr) into each corner space as prevously.
Continue in this manner throughout repeating Colours in this order observing that as the size of the work increases so, too, will the number of of ch spaces to be worked increase in number. When work has reached desired measurement for cushion make another identical piece.
With Right side of both pieces outside match the two pieces together. Using Colour Two sew tog three sides of the matched pieces using overhand stitch. Insert cushion, then complete fourth side.