The official text of treaties which Spain ratifies are published in the BOE Sección I. Disposiciones generales.
As well as producing its own legislation, Spain implements law from international organisations, including the UN and the European Union. Articles 93 - 96 of the Spanish Constitution clarify how international law is treated.
Any treaty that is signed, ratified and published in the Boletín Oficial del Estado automatically becomes part of Spain's national law:
"Any international treaty signed and ratified according to the rules of international law and this Consitution will be part of the legal system." Art.96(1) CE
However, in some cases -- if it requires financial contributions from the public treasury or requires additional legislation in order to be implemented, for example -- a ley orgánica (see the links below for an explanation of this term) or consultation from the chambers of Parliament may be required:
Art. 93 CE. A ley orgánica is required if the treaty involves the transfer of a state power to an international organisation. See also the Constitutional Court's decision STC 28/1991 de 14 febrero. BOE: 15/3/1991
Art 94(1) CE outlines the types of treaties which require the executive to get the authorisation of both chambers of the Cortes Generales before signature.
Art.94 (2) CE where a treaty does not fall into either of the above categories the government need only inform the Cortes Generales after ratification.
European Union legislation
Spain became a member state of the EU in 1986, and since then has transferred various powers to the EU. EU legislation is treated somewhat differently from other international law, as the Supreme Court and the European Court of Justice have resolved that any conflict between European and Spanish law will be resolved by ordinary jurisdiction, under the principle of the supremacy of EU law.
The procedure called el recurso de inconstitucionalidad allows the government or parliament to ask the Tribunal Constitucional to consider in advance whether the proposed treaty is constitutional. If the court rules that the treaty is in conflict with the Constitution, then the government would have to be achieve the modification of the Constitution before proceeding any further with the treaty negotiations.