easiest is probably to read the wiki for a few example.

Short version is that you need to create a macro to tell your slaves to target what your master is targetting.
using the TAB key to target a mob on all toons will not work well.

The macros are usually similar to what follows. Lets say your main is called Bob.
The macros (for the slaves only) would look like (all three achieve achieve the same thing)

1. this macro selects your main's target and casts a spell towards it
Code:
/assist Bob
/cast yourSpell
or
Code:
/cast [@Bobtarget]  yourSpell
or
(assuming you have your master as a focus)
Code:
/cast [@focustarget]  yourSpell
there's many more ways of doing this, all you have to do is understand the concept and then chose the way you prefer :-)

Obviously once you've created those macros, you'll need to drag them to your cast bar.