Each kind of initiative follows its own path through Congress. Pick one to see its steps, explained one by one.
A bill proposed by a parliamentary group. If passed, it becomes binding law.
Binding: if it passes, the law changes.
A parliamentary group or fifteen deputies table it. The Senate, a regional parliament or 500,000 citizens' signatures can also propose one.
The plenary votes on whether to take it into consideration, that is, whether Congress will start working on it. Before that, the Government gives its view and can object if it raises budget spending.
A committee (often a smaller drafting panel first) studies the text, negotiates and votes on the groups' amendments, producing a report. Sometimes the committee approves it outright and the plenary is skipped.
The plenary votes on the amendments the groups keep alive, then on the whole text. An organic law needs a whole-text vote by absolute majority: 176 votes.
The Senate has two months (twenty days if urgent) to pass it, amend it or veto it. If it changes the text, it goes back to Congress, which decides which changes to keep.
The King gives royal assent, it is published in the BOE and comes into force on the date the law itself sets.