Installing

This program is only available as source, but do not worry on linux you can get it running in seconds even if you have never tried this before.
On other systems it should be just as easy, but as I do not use them I offer no advice.

Download the source file.
Open a terminal and CD to the directory where the downloaded file is.
Enter the following command

gcc -o flv_trim flv_trim.c

Thats it you now should have a file called flv_trim which you can run.