#haml.download
First of all, you need to install the Haml gem. Most people will just want to use the most recent stable release. To get it, just run
gem install --no-ri haml
If you want to use the latest and greatest stuff in Haml, though, you'll need to install it from Git. To do so, run
git clone git://github.com/nex3/haml.git cd haml rake install
Rails
Once you've installed the gem, all you have to do to enable Haml for a Ruby on Rails app is run
haml --rails path/to/app
This will add a tiny plugin to your Rails app that loads the latest Haml gem and sets it up as a template handler. Then all .haml views (or .html.haml for edge) will be rendered as Haml.
This also sets up Sass for the Rails app. Sass files aren't views; instead, they go in public/stylesheets/sass. They're compiled to corresponding CSS files in public/stylesheets whenever a Rails page is loaded. For example, public/stylesheets/sass/main.sass would be compiled to public/stylesheets/main.css.
Merb
As long as you have the gem installed, Haml works in Merb without any extra tinkering. All views ending in .html.haml are automatically handled by Haml.
Sass isn't quite as automatic. If you want to use Sass stylesheets, you have to add
dependency "haml"
to config/dependencies.rb. Then it works just like Rails - all .sass files in public/stylesheets/sass are compiled to .css files in public/stylesheets.
Standalone
If you're interested in using Haml without Rails or Merb, just require "haml" and you're good to go.
Downloadable Archives
Available from Rubyforge.