@bmtanner +1 on above post - and also to emphasize again that you must install at the incoming Line end, not the load end 

I created this diagram for another member - hopefully that is helpful for you also

Note that you do not have to do anything regarding re-wiring of the 'load' end switch (as long as you have at least successfuly identified which one is which) 

And to confirm this will provide manual operation for on/off from either switch (just as it does now with the dumb switch config) as well as the Smart capability. 

@bmtanner Yes, either a meter or a non-contact voltage tester (which is a great tool to have around and not that expensive) - it will even read through the insulation, so very safe to use on a powered circuit. 

Do search for non contact voltage tester at Home Depot or Amazon etc 

Here on Amazon - non contact voltage tester

(they all do the same thing essentially, some just have more additional features beside the fundamental) 

@bmtanner You can sometimes tell, especially if one is a dual gang box - if there is common incoming black were that feeds TWO switches, then that is almost certainly a line wire (a load wire would not go to multiple switches). But still just safer to get a tester - it's a handy tool to have and especially if you plan to do more of these (and you will! It's addictive!) 

@bmtanner Here's an updated schematic including the ground and Neutral as White (as it should be) :)