Every ten years, as directed by the Constitution, the U.S. Census Bureau conducts an "actual Enumeration" of American residents. Upon release of the official population numbers, U.S. congressional seats are reapportioned among the states, depending on which states have gained seats and which have lost. State legislators then get to work developing new congressional and state legislative district maps to reflect both national and state-level population shifts over the past decade. In most states, redistricting generally comes down to a legislative battle like any other, requiring the input of the state's legislative bodies and the approval (or veto) of the Governor.
J. G. Hebert & Marina K. Jenkins,
The Need for State Redistricting Reform To Rein in Partisan Gerrymandering,
Yale L. & Pol'y Rev.
Available at: https://digitalcommons.law.yale.edu/ylpr/vol29/iss2/5