To get started making your virtual bridge and virtual network cards, you'll need to use the User Mode Linux utilities and the Bridge Utilities. Install these packages using your preferred method. On Debian I use the command

sudo apt-get install uml-utilities bridge-utils

Before you can create your virtual network cards (TAP interfaces) you need to load the tun kernel module and then change the tun device permissions such that all users can create TAP interfaces. Use the following commands

sudo modprobe tun
sudo chown root.vboxusers /dev/net/tun
sudo chmod g+rw /dev/net/tun

These changes to the permissions and ownership of /dev/net/tun will be lost after a reboot because loading the tun kernel module again will re-create /dev/net/tun with the default permissions. Also the tun module is not guaranteed to be loaded after a reboot, so we want to make the changes permanent by adding the correct lines to certain config files. As always, if we want to guarantee a kernel module is loaded (in Debian, and most distros are similar) we add the name of that module to /etc/modules so add a line with the word "tun" to /etc/modules. I always suggest putting comments with your name around any changes you make to these key config files so it's easy to grep for your name and takes your changes back out if you break something. Next, to make the ownership and permission changes to /dev/net/tun permanent, we want to add the two lines from above to the /etc/rc.local file. The only difference is that we don't have to put sudo before them, because the rc.local script gets executed with root permissions at startup. This should have your machine in the correct state to create TAP interfaces after every reboot.

Before you start setting up your bridge, make sure your network interfaces are all down. On Debian I use the command

sudo /etc/init.d/networking stop