A complete community document could be found at:
OK, let's get started. Run the following command:
sudo apt-get install virtualbox-ose
sudo apt-get install virtualbox-ose-modules-`uname -r`
sudo adduser $USER vboxusers
sudo apt-get install util-linux
The document mentioned something about module problem for kernel version 2.6.24-22 or higher, which is no longer a concern.
Then logout and relogin.
Install Ubuntu 9.04 in Virtualbox
If you have a multi-core CPU, start virtualbox this way. In your terminal:
taskset -c 1 virtualbox
This will force virtual box to use only one core.
In the virtualbox, click "New", then name your virtual machine. In the OS type menu, select "Linux 2.6".
Then you need to assign memory to your virtual machine. I give it 512M out of my 2G memory. And I selected dynamic size virtual hard drive. This is pretty much it. Other things are quite straight forward.
Before you start your virtual machine, in "CD/DVD mount" section on the right, mount the Ubuntu 9.04 iso, which can be downloaded here.
Then press start, and enjoy!