Turning two-way switches into three-way
14/2 is a cable.  It contains 3 wires if you count ground.  I will use those words that way.

On your existing switch, note the black that goes to the other blacks.  That is always hot. The other wire, mark it with red tape. That is switched-hot.  

Run a /3 cable from this switch to the other switch.  Mark the black and white wires with yellow tape on both ends.  Those are travelers. 

In this box, connect the /3 cable's red wire to the wire you just marked red.   

Wire up your 3-way switches as follows. 


the 2 yellow traveler wires go to the 2 brass (yellowish) colored screws on the 3-ways. 
now there is one wire still not connected in each box. That goes on the 3-way black screw.


And we're done. 

This simplicity brought to you by the maker of colored electrical tape.