Step 1
Scan in the left side, the right side, and the center of the spread. Make sure to have bunches and bunches of overlap in your center scan. The more overlap for both sides, the easier it is to recompose the image. Also, make sure the magazine is flat and the top of the page is flush with the top of your scanner.
Step 2
Create a new image. Make it large enough to fit your picture vertically, and give yourself extra space horizontally. I know from experience that if I scan a spread at 300dpi, I need a canvas that is 5500 by 3508 (my scanner's height is 3508 pixels.) You can crop the extra space later.
Step 3
Drop in the scans you've made. Put the center scan in the first layer, with the left and right scans above it. Try to put the left and right scans in the general area of where they should be overlapping, and obviously the center scan should be in the center of the canvas.
Step 4
Zoom in to the area where the right scan and center scan overlap. Set the right scan's layer blending properties to difference. Using this you can tell when the two scans are overlapping - the image will be black in those areas. Otherwise you'll see a variety of lovely, eye-blinding colors including fuschia, cyan, and lime green.
Step 5
Move the right scan until it overlaps as best as possible with the center scan. Unless you're really lucky, they probably won't overlap perfectly. As you can see here, while some portions match well, other portions are somewhat off.
Step 6
Transform the layer using a slight amount of rotation. You can either rotate it manually or enter in a number in the transform toolbar (see the 0.2°?) Move the layer around as necessary until you get a good fit. It may help to zoom in and out to make sure the layers match on both an up close and far away scale. If you need to rotate a layer more than 1.5° it may be better just to rescan that layer. Once it is to your liking, apply the transformation.
Step 7
When you set the layer blending properties back to normal, you should see the edges match fairly well. But there's probably an ugly line or border on the edge, because they're two different scans. To blend the two layers together, add an empty layer mask to the upper layer.
Step 8
Use the gradient tool (pure white/pure black) to create a soft transition between the two layers by applying it to the layer mask in the area of overlap. It may help to select the upper layer's transparency before adding the gradient to speed up the process (a gradient over 5500x3508 pixels takes a little while.) You should no longer be able to see the transition between the two layers.
Step 9
Repeat for the other side. When you're done, flatten the image.
Step 10
Crop those unsightly borders and resize as necessary. Save the image and wall. Or upload. Or whatever.
