Have you tried googling?

3 way switch diagram

Or if it's the 14-3 between the existing switch and the light that's throwing you off, here's a modified diagram that shows your exact situation.

The blue circled area is the new stuff that you'll add.

The red wire from the existing switch box to the light box can be left alone. Or it might be the black one. Depends on which wire is attached to the light. The wire that isn't attached to the light is just feeding power to the downstream objects and has nothing to do with the 3 way switch setup.

Note: You'll obviously also change out the existing switch for a 3-way switch

The diagram above also follows another rule, and that is, for every hole in which currents passes through, the net current must be zero to avoid a transformer effect and heating of the surrounding metal. This also means that the currents must be in phase to cancel each other fields. For any hole in the boxes above, the same current is entering and leaving through the same hole to give a net zero coupling.