Step 1: devise a sustainable system of growing food, where "sustainable" means that the process can be maintained for at least five hundred years without exhausting resources.

An example of unsustainable agriculture is a model based on mining phosphate.

An example of sustainable agriculture is a model where human waste can be converted into fertilisers (e.g. by processing sewage using reed beds, then charring the reeds to use as biochar and mulch).

Turn our attention to plants which will efficiently store phosphorous in a biologically available form, and ensure that the phosphorous we currently discard in effluent can be captured.

Treat spaceship Earth as a closed system and relise that we have to plumb our outputs back into our inputs or there will be a heck of a mess to clean up later.

There is also the issue of the begged question when asking, "how do we feed 9 billion people?"