The OP asks for a specific "step by step process to get a paper published by a deceased person who wrote the paper alone".
A statement on this was made a few years ago by COPE (Committee on Publication Ethics). This involved a case where the manuscript was submitted by the author, who passed away before it was accepted.
The first step is to ascertain whether the author intended their work to be published in the present form. Ideally the author has had a chance to respond to referees, but one might make the case that submission to arXiv suffices.
If this is ascertained, then one needs to obtain permission from the legal heirs of the deceased. Which documents are needed would depend on the journal, it should include at least a copyright license and for some journals also a statement of "no conflict of interest".
Point 2 should not be a problem, point 1 is more troublesome. Journals in general require a statement to the effect that "all authors must have read the paper and approve of it". It would be unusual for a journal to publish a manuscript posthumously if it was uncertain whether or not the author was ready for publication.