As has been stated above, broadcasting all is not 100% accurate, so the characters will desync quite fast. As such, broadcast is only used for movement in very specific situations. More commonly you would use it to click on portals, menus, quest dialogs, walk through teleports that would otherwise just break follow, etc.

The way multiboxing in WoW works (for the most part) is that you lead from one character, and the others /follow it. Similarly you acquire targets for attacks by /assisting your lead character. All of this functionality is however setup for you "out of the box" by ISBoxer, so you don't need to be setting up macros just yet. ISBoxer will automatically handle creating an FTL setup for you, so that the same keys for Follow and Assist work from any character (up to the max of... 24 characters I think?).

Mirai covered this among quite a lot of other things in his Multiboxing Gameplay video series, which I highly recommend for anyone just starting out with multiboxing MMOs.