Stitching a Layout

First, scan the image in 4 times: top left, top right, bottom left, bottom right:

Next, click on the first layer, and copy the whole layer. Copy the layer onto your clipboard:

Now, you're ready to make a new document that will eventually be your finished, stitched layout. Create a new document that's about twice the size of the layer you just copied. Go to file>new to make the new document. Remember, the size of MY document may not necessarily be the same size as YOUR document (especially since this one is an 8½"x11" layout!).

Paste the copied layer into your new document. Move the layer to the top left corner.

Then...

This is more or less how your new document should look at this point. Notice how part of the picture is in both of the layers. Next, we're going to stitch them to create a seamless look.

Now, select your Rectangular Marquee tool. Change the feather to 10 pixels.

Drag your rectangular marquee around the part of the 2nd layer that looks like it will overlap the first. Be sure to leave a few pixels' worth of space so you don't have any white space.

Next, hit delete. This will delete the area you selected.

Deselect the rectangular marquee (Control + D). Now we stitch! Move the 2nd layer over the first until you see that it all lines up nicely. When you do this, it's best to zoom to at least 200% so you can really see what you're doing and get the overlapping areas to be accurate. Select the Move tool (the arrow icon next to rectangular marquee). The next two pictures are more or less the same thing - the first is almost there, the 2nd is lined up correctly. You'll see how you don't see anything overlapping or any shading differences.

Be sure to look all the way from the top to the bottom to make sure it's lined up correctly! It might look correct on the top but could be 1 or 2 pixels off on the bottom. Notice how I'm working at 200%; click on the mountains icon circled and labeled "2" to make it 200% (#1)

Go to the 3rd scan you made (bottom left), select all, copy, and paste into your stitching document, as shown below. As you probably guessed, you're going to repeat above with making your rectangular marquee over the overlapping part of the layer, and then moving it so it fits with the rest without any seams.

3 out of 4 layers are complete! This is (more or less) how it should look now:

The 4th layer is a little tricky, because you have to feather out 2 different parts that will
overlap:

Now you're ready to put the last piece of the puzzle in! Once you've got it lined up right, select your crop tool and crop out all the excess white space. Draw a rectangle around the area you want to have, then double click in the light area.

Voila! Your finished product is a seamlessly stitched document.

If you don't care so much about making it seamless, you can do more or less the same thing except leave out the feathering. The reason I don't leave that out is because when I scan, the sides tend to be a little darker so when I overlap them, I get a dark line that I find distracting. You can also use just 2 scans, but if you're doing a 12x12, you'll be missing the top and/or bottom of the layout.