This assumes you have a means of broadcasting your keys and you know how to make macros and bind keys.

To get started as quickly as possible do the following:

Log in with all your toons

For each toon go to the keybindings window and bind the F and T keys to an action bar somewhere (I suggest showing the additional action bars and using them)

Open the macro window on each toon

Create a new macro called Follow and type this into the macro:

Code:
/follow focus
Drop the Follow macro onto wherever you bound the F key on the action bars

Create a new macro called Target and type this into the macro:

Code:
/assist focus
Drop the Target macro onto wherever you bound the T key on the action bars

Have your leader invite all your toons to a party

On each of your toons right click the party leader and select 'Set Focus' from the popup menu.

Press the F key and your toons will follow the leader.

Target a mob with your leader then press the T key to have each of the followers target the same target.

Cast spells on your main and your followers will cast their ones.

Once the target is defeated press the F key to have your followers follow the leader again.

You are done.