Is it as simple as "House proposes a Bill, and then everyone has to approve of it in the various branches, and then poof, it's there?"
In a way, it's even simpler than that. Election rules are defined on a state level. Depending on a state, ranked-choice voting can be implemented through legislative action and/or referendum on a state level.
Because, if so, why hasn't literally anyone proposed such a bill yet?
Maine has already switched to a ranked-choice voting system in 2016 through a referendum question 5 (An Act to Establish Ranked-Choice Voting).
Question 5: Citizen’s Initiative
Do you want to allow voters to rank their choices of candidates in elections for U.S. Senate, Congress, Governor, State Senate, and State Representative, and to have ballots counted at the state level in multiple rounds in which last-place candidates are eliminated until a candidate wins by majority?
Here's what the ballot looked like for a 2018 US Senate elections in Maine:
Proposing such a bill on a federal level would be unnecessary and
potentially unconstitutional, controversial, as it will impose election rules on individual states.
EDIT: As @Avi pointed out in the comments, such a bill is likely to be constitutional as the Congress has the power to regulate elections for Senators and Representatives (Article I, Section 4):
The Times, Places and Manner of holding Elections for Senators and Representatives, shall be prescribed in each State by the Legislature thereof; but the Congress may at any time by Law make or alter such Regulations, except as to the Places of chusing Senators.