The correct way is to run it using the cucumber executable if you’re using Rails 2, or bundle exec cucumber if you’re using Rails 3 (and thus Bundler).
To run a specific feature:
[command] features/signing_in.feature
To run a specific scenario from that feature:
[command] features/signing_in.feature:6
The line number can be any line inside that feature, but is usually the first line.
If you run rake cucumber:ok and some scenarios fail, at the bottom of the output you will see something like this:
cucumber features/sigining_in.feature:6 # Signing in via form
You can triple-click this line and paste it into your terminal to just run that scenario.