From the website:

Making the squares consistent in size and weight will help our wonderful volunteers (and grandmothers, grandfathers, mothers, husbands, brothers and sisters) join the squares quickly and easily to make wool blankets for the children and orphans of Southern Africa.

But as long as the square measures 8 x 8 inches (20 x 20 centimetres), you may like to be a bit more creative with your knitting. You can use different types of yarn, different knitting stitches, or perhaps use up scraps of lighter weight yarn, by doubling it up.

Wool will provide maximum warmth, but if you only have acrylic or blend yarns use those too, as any squares are better than none.

Please weave in the beginning tail (yarn end) and leave a 1 meter (1 yard+) length of yarn attached at the other end, neatly butterflied and laid on top of the square.