To create a symbolic link:
$ ln -s original/file new/symbolic/link
For example, assuming you are in a Rails app:
$ ln -s ../vendor/assets/swf/swf.file public
$ ln -s ../vendor/assets/swf/swf.file public/swf.file
would create a new symbolic link of public/swf.file to vendor/assets/swf/swf.file. Had I not specified the .., then the link would have not worked.
To test that you have the paths correct, you can do
$ file new/symbolic/link
If you 'broken symbolic link to ...' in the output, make sure that you specified the original file path relative to the directory that the file will live in.