The method depends on the software used, and the type of finished artwork desired. I run Linux, so I have Windows XP in a VM to run ExpressPCB. By nature of the application, the bottom layer is always printed correctly. The top layer, if there is one, must be reversed. Since this software is meant to be used to order directly from ExpressPCB, you can't just use the normal output for this. What I do to make a single sided PC board is print the bottom layer to the laser printer, which is an HP P2035 in my case. In the print dialog it asks me to select a layer, and I select only the bottom copper layer. The result is a full size, properly oriented positive image of the bottom layer of the PC board.
An alternative which I use often is to print to the "Generic Postscript Printer" in Windows XP, and when asked for a filename, I enter "blah.eps" where "blah" is the name of the ExpressPCB file. The resulting file may be loaded into any vector drawing package, like Inkscape, and printed from there after manipulation. For example, to print 2-up images, I print to an ".eps" file and then load the file into Inkscape and select it, copy it to the top, paste another, move it, etc.
If you use Gimp, which I generally do, you would import the vector eps file at 1200 DPI, grayscale. Once it is in, crop as appropriate. Use Color->Invert to turn the image into a negative, if required.
If you are going to use the negative or positive photoresist methods, you would print the file onto transparency film. If you are going to use the laser film method, you would print the positive file onto the resist film. If you are using the laser toner method, you would print to the paper type you have chosen.
Summary of Steps
1.Print from the PC board layout software to a Postscript file.
2.Open the Postscript file in Gimp and import it at 600 DPI, grayscale.
3.If the image is the top copper layer, Image->Transform->Flip Horizontal.
4.Crop the image to about 1/4 inch larger than the PC board.
5.Invert the image colors if a negative is required.
6.Load the appropriate media into the laser printer.
7.Print the image to the laser printer.
8.Artwork preparation is complete.
