I have seen this answer: How do I convert a light circuit with a single pole switch to use two 3-way switches?
However, I'm having trouble understanding what my current circuit layout is (I didn't install it) and haven't been able to get three-way switches working.
Here's a diagram I made of what I can understand of the circuit:
Sorry for my poor, incomplete diagram. Both cables run over ten meters through the ceiling. I can't see where they go to even know where I'd need to crack the ceiling to find them. Also, I lack enough knowledge to complete the circuit logically.
If anyone could help me I'd appreciate it a lot.
EDIT: Adding a picture of the current wiring diagram. This connects the circuit only if both switches are on. Either switch breaks the circuit.
EDIT 2: Switch Wiring Pictures
Switch closer to the lights on the circuit:
Switch closer to the source: