How to combine multiple flv files into one flv file

While downloading video from YouTube is very easy nowadays because of various tools available to help us to do the job, downloading video from other websites still not easy, especially YouKo (优酷, a famous YouTube in China). The problem I have is the flv file saved from YouKo is always in multiple files (each file for 7min~). Here is a tutorial on how to combine multiple flv files into one flv file.