Each kind of initiative follows its own path through Congress. Pick one to see its steps, explained one by one.
A bill proposed by the Government. If passed, it becomes binding law.
Binding: if it passes, the law changes.
The Council of Ministers approves the text and sends it to Congress, with a report explaining why.
It is published in the Cortes' official gazette and the groups can table amendments. If a group asks to send it back (amendment to the whole), the plenary votes on that first.
A committee (often a smaller drafting panel first) studies the text, negotiates and votes on the 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.