Does your printer uses hardware handshaking as well? If so, then short the RTS to CTS (7 to 8 on DB9), DSR to DTR (pin 4 to 6 on DB9) on the PRINTER side if the printer uses DB9.
You mentioned that your printer uses DB25, then for DB9 to DB15 wiring, it depends on whether your printer is configured as DCE or DTE. So it may be 2-2, 3, 5-7 or 2-3, 3-2, 5-7.
If your printer uses strictly XON/XOFF protocol for flow control, then please check with the printer's supplier on the protocol spec. My guess is that you may have to send an XON character to the Printer before sending the data. Find out what ASCII character they use as XON and you can use the CHR$(n) to send the character to the printer prior to the data.