The maths diagnostic will start automatically for all new pupils when they first start using Sumdog.
If you have pupils who you would like to have retake their maths diagnostic here is how you can reset their score. Once their scores have been reset they will be able to take the maths diagnostic.
If you have set any work for your pupils, like a challenge or assessment, they will need to select their maths diagnostic from their Tasks list.
These instructions explain how pupils can take their maths diagnostic.