I must apologize, I see that the posted manual does not have the schematic, like the "old style" manuals.
This one is representative:
http://www.accucraft.com/manuals/AT 20p3 SP 4-6-0 EL LS Web Manual.pdf
But I have NEVER found that any Accucraft schematic was accurate. A friend and I were working on this particular loco, and the actual wiring was still different, and more similar to other models.
The bottom line is what good advice I can give you: some the LEDs already have a dropping resistor inline, a very tiny one, usually in heatshrink near the LED itself, and these are sometimes 1/8 watt resistors, basically the same diameter as the wire with insulation.
Most of the smaller lamps, headlights are given 6 volts regulated, and with the dropping resistor inline draw something like 10 milliamperes or less. When we connected these to the 5 volt common on a decoder, there was no appreciable loss of brightness.
The marker lamps were often wired in series and drew more current, but on some locos they were incandescent and powered by a 3.5 volt regulator and on some were 6.5 volts. You should measure the voltage....
The cab lights, when present were a crapshoot.
In the tender there are usually 2 boards, identical boards that vary by the number of sockets soldered in. on the underside of each board there can be up to two 3 terminal regulators, and if you put the tender on a track, run it up to 12 volts, you can measure the regulated voltages supplied... this is the safest way NOT to burn out those small hard to replace LEDs.
I did have one loco where the 6 volt regulator was hidden on the underside of the gear tower in the loco itself... that was a bugger.
If I can be of any more help let me know.
Greg