What are the conditions and rules for someone to declare his or her assets as Waqf? Besides, does it apply for intellectual properties?
If a person makes something Waqf, it ceases to be his property, and neither he nor anybody else can either gift it or sell it to any person. Also, no one can inherit anything out of it. Source
If you can understand Arabic there is a scholar whose name is Abdullah bin Bayyah. He made a lot of research in this topic. He also has an english version of his site but I don't think all of his researches are translated to english unfortunately.
I'm not a scholar but I'm interested a lot in this topic.
For your second question, from bayyah's research, I remember that intellectual properties were considered as waqf because it produces money as the publisher can buy and sell these intellectual properties of books.