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

Have a look at this site. Watermark Your Videos and Images (http://www.deskshare.com/Resources/articles/vem_watermark_videos.aspx)

Here are some download sites as well.
Watermark Your Videos and Images (http://www.deskshare.com/Resources/articles/vem_watermark_videos.aspx)
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 (http://webscripts.softpedia.com/script/Multimedia/Video/FLV-Flash-Video-Watermarking-39772.html)
Free Watermark software: Download Watermark (http://www.newfreedownloads.com/find/watermark.html)
I hope this helps you out. Best of luck