Full Idea
When Kant said that existence was not a property, what he meant was, according to Frege, that existence is not a first-order property - it is not a property of individuals but a property of properties, that the property has an instance.
Gist of Idea
Existence is not a first-order property, but the instantiation of a property
Source
report of Gottlob Frege (Begriffsschrift [1879]) by Stephen Read - Thinking About Logic Ch.5
Book Reference
Read,Stephen: 'Thinking About Logic' [OUP 1995], p.126