flex - how to generate my first filter

[Joe]

How do I use it? I've read the help at gnu.org, but it neglects to explain
the details to the complete novice.

I've run it from the command line but it just hangs and does nothing.

If I want to produce a filter that just outputs what it reads, then,
apparently, the 'rules file'(?) should just have the following in it:

%%

Am I supposed to create a text file with %% in it and then specify that file
on the command line?  How Exactly?

Once I can do this, then I'll probably be able to work out the rest.

Thanks.
(aware of x-posting)