I have just installed Ubuntu Dapper Drake Flight 6 on my desktop machine, and because I had had different problems to install Rails from scratch several times (even the recent session was no exception), I have decided to write a step-by-step guide, which assumes a clean, fresh install of Ubuntu ( i.e. at this point you do not even have Ruby on your machine) and leads you through installing Rails and creating a working test application.
Why is this writeup better than any other how-to-install-rails tutorials out there?

• Because it will tell you to install really just what you need, not 50 packages more
• It will also show you how to configure the DB and other things to really make Rails work, not just installed

Let’s get started!
Note: Some people asked if this manual is for dapper only. I would say mostly yes, because i have had different problems on breezy (for example i had to compile ruby-mysql driver manually). Its not entirely impossible that it will work with breezy – but then you will have to make sure that the packages are the same version as assumed here (e.g. MySQL > 5 etc.)

Part I: Installation

Prepare the system for the installation

• Check /etc/apt/sources.list – make sure you have access to the ‘universe’ packages by uncommenting them:

deb http://us.archive.ubuntu.com/ubuntu dapper universe
deb-src http://us.archive.ubuntu.com/ubuntu dapper universe

• Refresh apt packages to make sure you get the most up-to-date stuff:

sudo apt-get update

Install Ruby related packages

• Install Ruby essentials: ruby, irb, rdoc, ri

sudo apt-get install ruby rdoc ri

• Install gems: download, unpack, install

go to http://docs.rubygems.org/
download rubygems-0.8.11.tgz (or the latest version)  tar -xzvf rubygems-0.8.11.tgz
cd rubygems-0.8.11/
sudo ruby setup.rb

MySQL installation and configuration

• Install MySQL:

sudo apt-get install mysql-server

• Install ruby MySQL bindings

sudo apt-get install libmysql-ruby

Install Rails

sudo gem install rails --include-dependencies

Part II: Configuration

Setup the DB

• Add an user, create a test database and grant acces for the user

mysqladmin -u root create test_development

mysql -u root

Into the db shell, write the following commands:

create user 'batman'@'localhost' identified by 'robin';
grant all on test_development.* to 'batman'@'localhost';

Don’t forget to replace the username/password (unless you happen to be Batman of course – in this case i suggest to use a different password since this can be guessed easily by social engineers 😉

Create and test the rails app

• generate the app files

Lets denote your working directory (the root directory where your future rails project s will reside rails_projects).

cd rails_projects
rails test

• edit config/database.yml

cd rails_projects/test 
vim config/database.yml

• It should look like this:

development:
adapter: mysql
database: test_development
username: batman
password: robin
host: localhost

• generate a dummy model

ruby script/generate model Dummy

• edit the migration file

vim db/migrate/001_create_dummies.rb

class CreateDummies < ActiveRecord::Migration
def   self.up
  create_table :dummies do |t|
    t.column :foo,    :string
    t.column :bar,    :string
  end
end

def self.down
  drop_table :dummies
end
end

• run the migration

rake db:migrate

• generate a simple maintenance app

ruby script/generate scaffold Dummy Admin

• start the server

ruby script/server

Point your browser to http://localhost:3000/admin to see the result.
If you have any problems, please leave a comment, i will try to help you.