As whykap says, build them backwards. I always sketch these out on a sheet of paper to organize the call flow and then build backwards (destination has to exist before you can point something to it). Using digits 1 through 9 you can provide up to 9 options per menu level, though best practices recommends no more than 4 or 5 options per level to avoid confusing your caller. If there'll be several options, say 5 or more, I recommend announcing that in the greeting, something like "please choose from one of following six options" and then enumerate them for the caller, ie for the service department press 1, for sales press 2, etc. Also keep the verbiage extremely brief also to avoid confusing the caller. -just present the list of options being as terse as possible. (ie, never-ever more than 10 words per option and the fewer the better).
Some of the best documentation on building call handlers can be found in the help screens.
Original MUG/NAMU Charter Member