I tried several sources to learn about this, and I still don't understand it thoroughly at all. The source that worked best for me was "Survey of gravitational radiation theory," F.A.E. Pirani, in Recent Developments in General Relativity, Pergamon, 1962, p. 89. You can presumably find it if you have access to a large university library. It's very concrete and down to earth, and it relates the ideas to their counterparts in E&M.