The decision-making process

The world heritage committee (21 States Parties) decides which new areas or objects are to be added to the world heritage list at their annual committee meeting. This is based on nominations by the member countries.

If the new nominations fulfil the technical requirements for content they are transferred to the International Union for Conservation of Nature (IUCN) and the International Council on Monuments and Sites (ICOMOS) which is responsible for the technical evaluation. These advisory organs make a written a technical recommendation to the World Heritage Committee which then makes the final decision. The evaluation process takes 18 months and involves field visits and discussions in international science panels that are appointed to look at essential issues in the application.
You can read more about the requirements for world heritage areas on http://www.dirnat.no/naturmangfold/internasjonalt/avtaler/verdensarvkonvensjonen/