You can find shapefiles from the US census:
Shapefiles and kml files are both available for congressional district, counties and other categories used by the census.
Some historical data is available from https://www.census.gov/geographies/mapping-files/time-series/...
The process is spelled out in the Administrative Procedure Act. See, e.g., National Urban League v. Ross and Ohio v. Coggins. Basically, it amounts to a lawsuit against a federal government official in the chain of command involved in making the decision in question filed in a suitable U.S. District Court.
The proper way to go about it is detailed by ohwilleke's answer. But for the sake of amusement let's ignore that and consider the fallback:
Reapportionment due to the Census, while required by the constitution, is not automatic and self-executing by default. It requires an act of Congress to determine both how many seats ...
100% turnout, Identical sized districts, Infinite population = 25% minimum.
E.g. 1200 voters in 12 districts:
49 blue votes, 51 red votes in 7 districts
100 blue votes in 5 districts
Total 843 blue voters, 357 red votes. (29.75% to stay in power)
1.2 million voters in 1200 districts:
499 blue votes, 501 red votes, in 601 districts
1000 blue votes in 599 ...