There is no "official" wiring diagram that I know of. For the locomotive side, I just wire the IR LED to the serial data with a series current limiting resistor, I used 1K for my Yellowbelly. If the load causes other serial data issues, I use my Serial Data Buffer to solve that problem.
In the tender side, assuming you're using the ERR Railsounds Commander board, I use the AD-20A stand-alone board. This has an IR receiver, serial data output, and also handles the backup lights and couplers. It does require a source of 5VDC, I add a small power module for that task.