Most of the images on this site are made using the Digitized Sky Survey (DSS2) or SuperCOSMOS.
Generally speaking, DSS2 is best for northern hemisphere objects (declination greater than 0) and SuperCOSMOS is better for southern hemisphere objects (declination less than 0). (The DSS2 archive is missing blue plates for most of the southern hemisphere and SuperCOSMOS has other advantages as well as described below.)
For northern hemisphere objects, visit the Digitized Sky Survey site and enter the name of the object in the search box on the left. Then select the POSS2/UKSTU Red image. Set the height and width to 60 arcminutes (i.e. one square degree) and the file format to GIF. Then click on the "RETRIEVE IMAGE" button.
It will likely take several minutes for the image to appear. When it is finished, save it to a file (something like "object_name_red.gif"). Then go back using the back button on your browser and select the POSS2/UKSTU Blue image. Click on the "RETRIEVE IMAGE" button, and save the blue image once it has finished downloading. Repeat once more for the POSS2/UKSTU IR (infrared) image.
The next step is to combine the three grey scale images into one colour image. Obviously use the Red grey scale image for the Red component and the Blue grey scale image for the Blue component. Use the IR (infrared) grey scale image for the Green component.
This creates a false colour image, and helps explain the pink clouds visible in many HII region images on this site (they are normally dark red in "true" colour images) as well the occasional green star or cloud. The advantage of using infrared in this way is that it lets you see stars, star clusters and nebula that are normally masked by clouds of dust.
The images will usually not just neatly stack on top of each other as they have slightly different scales and alignments, so I suggest that you use an astronomical image processing program to combine the images and bring out subtle nebula. I recommend CADET. It's free and has more than enough features to make good pictures. I suggest that you align the images on two stars for maximum quality, and then use the histogram functions to bring out faint nebula.
CADET does not accept very large images so you may need to resize the DSS grey scale images (I usually reduce the width to 1024 pixels) before you align them.
After I save the resulting composite image from CADET, I import it into a standard graphics program. I personally use Paint Shop Pro, which is not free, but you can also use the open source Gimp package. Fiddle with the brightness and contrast to further highlight faint nebula. I then crop the image as desired and shrink it to 360x360 pixels.
I know it's a shame to reduce the resolution so much, but I want to give visitors quick peeks at a lot of objects and I need to conserve precious bandwidth and disk space.
The process for imaging southern hemisphere objects using SuperCOSMOS is much the same. This site, run by the Royal Observatory in Edinburgh, has blue plates for almost the entire southern hemisphere. Even better, these plates have been scanned at a higher resolution than the DSS2 images, and hydrogen-alpha plates are available as well. As hydrogen-alpha is the deep red colour given off by ionised hydrogen gas and so is the precise colour of most HII regions, these plates provide an unprecedented level of detail for nebulae. I first combine the H-alpha and red plate images, as suggested by this website, before creating a full colour composite image as described above.