1. Get a majority in the Senate and House. (This is perhaps the hardest part, but it may happen someday.)
2. Some blue states pass bills that allow each of their congressional districts to petition Congress for statehood.
3. Some congressional districts petition for statehood.
4. Congress grants statehood to many Congressional districts.
The Constitution is clear on the statehood process: “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.”
These new states would be about the size of Wyoming, Montana or DC.
This would likely kick off a process where more state would break up. If the Republicans got control of Congress then they would probably retaliate by breaking up red states.
There is nothing that would keep these small states from creating legal confederations. States already create legal confederations.