To convert video files to FLV format you will need a conversion application. To use a converter, you simply import a file, choose a preset, and then export as FLV. There are many converter/encoders. Some of the most commonly-used converters are:
Flix : Flix is a collection of encoders by On2 Technologies, the company which makes the VP6 codec used in Flash 8.
Any DVD Converter Professional : A free program which will convert any DV, DVD, RM, MOV, VOB, MPG, or MP4 video format to FLV.
Sorenson Squeeze : A powerful and versatile conversion application.
Adobe Flash : The official Flash authoring program comes bundled with a utility called Flash CS3 Video Encoder. Adobe Flash also comes with the FLV QuickTime Export plugin which allows you to export FLV files directly from the following video editing applications: