The short answer is that Congress passes a federal law. The admission of new states is governed by Article IV, section 3 of the Constitution, which reads:“New States may be admitted by the Congress into this Union; but no new State shall be formed or erected within the Jurisdiction of any other State; nor any State be formed by the Junction of two or more States, or Parts of States, without the Consent of the Legislatures of the States concerned as well as of the Congress.Although various detailed plans for structuring the statehood process were considered both before the adoption of the Constitution and as revisions to it in the 19th century, the Framers went with a plan that did not constitutionally constrain either the size of new states or set a population threshold required for admission. Nor did they require any supermajority procedures. In effect, they made it surprisingly easy to add new states. Creating a new state is arguably the only irreversible process in the entire Constitution. Yet, it requires no more than federal law to achieve it.
The Congress shall have power to dispose of and make all needful Rules and Regulations respecting the Territory or other Property belonging to the United States; and nothing in this Constitution shall be so construed as to Prejudice any Claims of the United States, or of any particular State.”
And it was immediately and always controversial. Throughout the 19th century, statehood played a prominent role in destabilizing American politics. New states' ability to alter the balance of political power in the federal government led to near-constant political jockeying over state admissions, as partisans sought to expand the Union as a mechanism of consolidating political power. Frustration with the process led to routine calls for its reform, especially for the imposition of size and population threshold restrictions on Congress's ability to add states.