How do I insulate a strip foundation from below?
I have doubts that you are expressing yourself clearly or if, so , that you grasp the concept that concrete does not mind water a bit (if you are just building a wall, not a foundation that encloses a buried space as normal for a building, where is the water going to go that causes you a problem, down in the ground?)

In any case, you can lay a sheet of plastic on the base before pouring concrete. But I seriously doubt you need to do that or any other waterproofing, if you are just putting in a foundation for a free-standing wall. If that's not what you are doing, have another try at explaining what you are doing, please (edit your question) since that appears to be what you are describing with "strip foundation" 

If you are building a normal, space-enclosing foundation wall, you can also lay plastic or EDPM rubber under the entire floor area, but the main thing is to lay drainage pipe outside the foundation, slightly below the floor level to remove water from the foundation area - depending on films or coatings alone is prone to failure against high water pressure - removing the bulk of the water is the most effective method to prevent water in the basement.