Generally this is done by whomever produces your PCB. You supply them with the data file, and they produce a set of "artworks", both positive and negative, for etching and for masking. If you are going to plot your gerber files on a printer, and etch your own boards then follow the instrutions bellow
According to OrCad themselves:
Creating negative image prints of copper layers to create board prototypes
PROBLEM:
In Layout, how do I create negative image prints of my copper layers to create an in-house prototype?
SOLUTION:
From within Layout, choose the Post Processing toolbar button. From the menu that displays, choose Setup Batch and enable Extended Gerber output for your layers. If you are hand-drilling the board, select the Keep Drill Holes Open option to leave an opening in the center of your pads as a drill target. Run batch post processing (choose the Post Processing toolbar button and then choose Run Batch) to create your extended Gerber files and your GerbTool .GTD design file.
From within GerbTool, open your .GTD design file. Double-click on the rectangular Draw Color block and set it to white. Then, double-click on the rectangular Flash Color block and set it to white as well. From the File menu, choose Page Setup and select the Print Background and Color/Gray Scale options. Make sure the Overlay and Sketch options are not enabled. Finally, print the layer by choosing Print from the File menu.
Document number: LAY01851
Applies to: Layout (v7.10, v9.0)
Created on: Monday, August 17, 1998
Last modified: Monday, August 17, 1998