Connecting the System/Peripheral 8 cable (Apple Part # M0197) to the LocalTalk port (Din-8) is a better solution because you can use the standard LaserWriter driver. This doesn't provide you with asynchronous communications to the printer, but using the standard LaserWriter printer driver is the method Apple recommends.
If you want to use the RS-232 port to use PostScript in the interactive mode, here are the pinouts:
Macintosh (Din-8) LaserWriter Pro (DB-9)
----------------- ----------------------
Signal Pin# Pin# Signal
GND 4 ---------- 5 GND
TxD- 3 ---------- 2 RxD
TxD+ 6 ---------- 1 DCD
HSKo 1 ---------- 4 DTR
RxD+ 8 ---------- NC
RxD- 5 ---------- 3 TxD