Just paste this URL to your browser address:

http://api.twitter.com/users/show/[your Twitter username].json

be sure to change the Twitter username accordingly (without the [ and ]).

When browser prompt for download, just open it with a notepad. You will see a bunch of text.

Find something like this;

“id_str”:”331383958″

331383958 is the ID.

Now if you would like to get the RSS for that user, use this:

http://twitter.com/statuses/user_timeline/[REPLACE_TWITTER_ID].rss

(again, without the [ and ]).