This will be a Jotto game for one player. See the Wikipedia article for details on the rules.
A typical interaction might look like this:
I'm thinking of a five-letter word. Guess what it is.
Guess 1 of 10: think
1 jot.
Guess 2 of 10: burnt
2 jots.
Guess 3 of 10: xyzzy
Nothing. Try again.
....
You're out of turns! Better luck next time.
(or)
That's it! Great guessing.
On the command-line (assuming you have rvm installed):
$ rvm gemset create jotto
$ source .rvmrc
$ bundle
(If this gives an error, you may need to run gem install bundle first.)
Now, if you will be working on lib/document.rb, you can run the tests like
this:
$ rspec spec/document_spec.rb
Pick one of the failing tests, figure out why it's failing, and fix it.
Lather. Rinse. Repeat.