Found this - I haven't read it all, but trust it will do:
To make the blancmange you:
Empty the powder into a jug, add a few teaspoons of sugar and a few tablespoons of cold milk and mix into a paste. Pour the rest of one pint of milk into a saucepan and heat up the milk. When the milk in the pan starts to get warm, you tip a little of the warm milk into the blancmange paste in the jug, then you add a little more milk as the milk gets hotter. Then you add the rest of the milk into the blancmange paste in the jug and stir it all then transfer the whole thing back into the saucepan.
Then it could all go wrong- so you have to stir the liquid in the pan constantly whilst you are bringing it to the boil. I mean constantly- your arms will hurt if you aren't used to this. Do not let it boil, but reduce the heat to a simmer when it starts to boil. Nothing will happen for about three minutes then all of a sudden, the mixture will thicken up magically- then you know it is done. You have to then pour it into your bowls and leave it to cool.