Wash the spinach leaves and shake off excess water.

If you are using large spinach leaves, it is better to remove the stems. You can use this handy device to make this job easier.

Dry the leaves and lay on a tea towel.

If using the oven, transfer the dried leaves to a lined sheet pan and set the oven to the lowest setting. Keep the door open a tiny bit to make sure the air circulated properly. Dehydrate the leaves by leaving them in the oven until crisp. Check on them every hour to make sure they are OK. Leave them for up to 4 hours or until crisp. Towards the end of the cooking time, check on them more regularly. 

If you are using a dehydrator, lay the leaves on a dehydrator sheet and dehydrate until crisp for about 8 hours at 110 F/ 45 C.

Blend the dehydrated leaves in a blender until pulverized.

Sift the powder with a mesh strainer to ensure uniform consistency. You can blend the larger chunks again until they also form a powder.

Store in a mason jar out of the fridge. This powder should last a long time as long as the leaves were bone dry when you blended them.