it's very easy.
1. make sure you have "rbenv" and a ruby version (e.g. 1.9.3) installed in remote server
2. in your config/deploy.rb (capistrano file)
3. in your Gemfile:
gem 'capistrano-rbenv', '1.0.1'
that's it. to debug if it works, just run:
$ cap shell
cap> which ruby ** [out :: 10.103.13.74] /root/.rbenv/shims/ruby cap> ruby -v ** [out :: 10.103.13.74] ruby 1.9.3p327 (2012-11-10 revision 37606) [x86_64-linux]