Always use tender spinach leaves as big leaves are known to cause stones. Also small leaves tend to be fresher and therefore are more healthier. After you tail the leaves put them in a big bowl of water and leave to stand for a few minutes.
The extra mud will then sit at the bottom of the water. Now wash them under running water.
Put all the washed leaves in a large vessel and put over a fire. Allow to cook for five minutes and cool. Run it through a mixie and your spinach puree is ready.
You can also store this in the deep freeze for use a few days later.
Spinach leaves are difficult to store so this is better.