1. Install Ant. Instructions for this can be found here
2. Download Adobe Flex 3 SDK. You can download this here.
3. Unzip the Flex 3 SDK that you just downloaded.

   % unzip flex_sdk_3.zip
   

4. Download the Flowplayer development kit. The latest version of the development kit can be found here.
5. Unzip the Flowplayer development kit.

   % upzip flowplayer.devkit-3.0.3.zip

6. Edit the plugin-build.properties file in the Flowplayer development kit. Specifically edit the line that says:

flex3dir=/Users/api/flex3sdk

to point to the location to which you unzipped the Flex 3 SDK. After editing my file looks like this:

flex3dir=/Users/anil/Desktop/flex_sdk_3

7. Navigate to the example plugin that is contained within the Flowplayer development kit. For me this is located here:

   /Users/anil/Desktop/flowplayer.devkit/example

8. Finally, type:

   % ant

   The output should look like this:

   Buildfile: build.xml
   
   build:
        [echo] ++ flowplayer.helloworld.swf +++
        [echo]
        [echo] checking if flowplayer.helloworld.swf is uptodate
        [echo] main up-to-date: ${uptodate.main}
   
   prepare:
       [unzip] Expanding: /Users/anil/env/flowplayer.devkit/flowplayer.swc into
       /Users/anil/env/flowplayer.devkit/example/build
   
   check-compile-dirs:
   
   check-compile-dirs2:
   
   compile:
        [exec] Loading configuration file
              /Users/anil/Desktop/flex_sdk_3/frameworks/flex-config.xml
        [exec] /Users/anil/env/flowplayer.devkit/example/build
   /flowplayer.helloworld.swf (1304 bytes)
        [copy] Copying 1 file to /Users/anil/env/flowplayer.devkit/example/build
   
   BUILD SUCCESSFUL
   Total time: 3 seconds
   

Congratulations! You have compiled a flash plugin for Flowplayer!