What documents do you need to issue a visa to China?

David Gubb, if you attend a conference in Beijing for two weeks, travel to Pusan in South Korea to visit a colleague for 4 days in the middle of that time, then return to Beijing, you should apply for a double-entry F visa to China.

The following documents are required:
your passport, complete visa application form V.2011A and V.2011B, one passport photo, the invitation letter from the conference organizer, the flight tickets

You need to travel to Madrid to submit visa application to the embassy. If you can't come in person, you can ask someone else to do it for you, but you can't mail your application to the embassy.