very simple
in your Gemfile add
gem "erb2haml", :group => :development
then run bundle install
for Converting *.erb to *.haml keeping original files do:
rake haml:convert_erbs
for Converting *.erb to *.haml replacing original files do:
rake haml:replace_erbs
it'll search all the erb files in project and convert to haml.
For shorthand: use on-line converter
http://www.htmltohaml.com