What you need: mono (not stereo) cable (see below)
What you get: a PPM signal with a 3VDC logic.
How to do it?
On your TX15:
Switch it ON and press the "MDL" button
In your "General Settings", select "Trainer"
On the next screen, choose "Slave/ Jack"
You should see the screen below.
That should output a PPM signal with the parameters displayed above. If unsure, check the output with an oscilloscope. Move a stick to see the pulses (channels). By the way, it's not necessary to modify any timing.
Notes:
CH1: it's the Roll stick.
CH2: it's the Pitch.
CH3: it's the Throttle.
CH4: it's the Yaw.