How do you watermark a vedio? I mean which software is best

Have a look at this site. Watermark Your Videos and Images

Here are some download sites as well.
Watermark Your Videos and Images
Download FLV Flash Video Watermarking - FLV Flash Video Watermarking allows you to stamp any FLV file with your domain name or any other watermark. - Softpedia
Free Watermark software: Download Watermark
I hope this helps you out. Best of luck