1- Download the FLV video with DVDVideoSoft Free Studio.
Comment: other tools will download AVIs in MPEG-4 V2 format, but I had problems later on manipulating the video without the codec (MP42 is the 4-digit code I think).
2- Convert the FLV to AVI and XVid or DivX encoding with the FLV Converter software.
3- Create the subtitle offline in a text file.
4- Open the video and the text file with DivXLand Media Subtitler. That's the easiest subtitler tool in the world.
Comment: it didn't work for me on Windows Server 2008 R2 64 bits. Not sure about Windows 7 64 bits... It only worked on Windows 7 32 bits for me.
5- Open the video with VirtualDub. Download and install the Alpha Station Subtitler plug-in for VirtualDub if you haven't done so yet.
6- Choose full processing mode in video in VirtualDub. Under 'Video -> filters' choose the subtitler filter (it will be there if you installed the plug-in in the previous step)
7- Save the AVI VirtualDub. The result is a new AVI file with the subtitles.
I had a problem with the subtitle in that the font size was too big. Size 48 to be precise. I manually opened the .ssa file generated by DivXLand Media Subtitler and I replaced font size 48 with 16 near the header of the file. It is a text file and it's not difficult to understand.