Taking the photographs.
A tripod is essential here as you want to keep the horizon in the final image level. Ensure the tripod head is level using a hotshoe spirit level or maybe your tripod already has one built in? Also, mount the camera in portrait format to maximise the resolution of the final image.
Take a test shot of an average part of the scene ie. not the darkest or lightest, with your polariser off if you normally have one on. If, after checking the histogram, the exposure looks OK switch to manual mode & dial in these settings. In addition, change the WB to “daylight”.
Ensuring you have an aperture providing sufficient DoF to leave everything in focus (f/8 – f/11 is often fine) hold your hand in front of the lens pointing in the direction you’re going to shoot. This will usually be left as for most people it’s easier to shoot left to right. Begin taking pictures with a 30-40% overlap (this enables the software to stitch them together easier later on). After the last image, take another with your hand in front of the lens but pointing right this time. Now you’ll have a clear sequence of images to link together when you get home.
Stitching the images together.
In Photoshop Elements 12 open all the files in ACR. “Select All” so that any adjustments you make apply to all the images. Now save them as jpegs & place them in a folder somewhere easy to find. Go to Enhance>Photomerge Panorama in the photo editor, leave as “Auto” or try a different style as you please & browse to locate the files you’ve just edited. Highlight them all & click “OK”. Elements will now do its thing (which may take a while depending on how many images you took in the first place). Once complete, you’ll probably get a message asking you to clean the edges. Either click “Yes” for the computer to fill in the blank areas or, “No” to crop the stitched image yourself. If you went for the computer option you may find you need to tidy up the corners. If this is the case, use the lasso tool to ring the bits that look wrong & then “Ctrl + Alt” to grab a section of the picture & bring it down ontop of the area to be fixed. “Esc” to get rid of the Lasso.
Finally, the new stitched image will be huge so Layers>Flatten image before saving.
NB. If you’re using the latest version of Lightroom or Photoshop CC you can stitch directly from ACR. Click the filmstrip & select “Merge to Panorama”, choose your style & click “Merge”.