How do I rebuild my fallen arches in my feet?

Really can't. It's not really possible to "rebuild fallen arches." if symptomatic, orthotics can "accomodate" a flat foot. Surgery options to rebuild an arch exist but should only be considered if symptomatic and failed non-surgical management.
You can't. Without surgery. This is only necessary in severe or traumaticly caused cases. Orthotics and proper shoe wear usually provide the needed stability and support to control or at least improve any related symptoms.