As movies have indicated, aliens have a huge brains so you enlarge the top of the person's head. Pick the Bloat Tool and click on the middle of the forehead. Now hold the left mouse button or click repeatedly, whichever you prefer, the effect is the same. You will notice the top of the head getting bigger and bigger. Stop when you are happy with the result. Here is what I have so far:

Next we will need to shrink the chin. Select the Pucker Tool which has the opposite effect than the Bloat Tool. Place the brush on the person's chin and hold left button or click repeatedly until you have shrunk the lower part of the head to the appropriate size (for an alien:).

Now you will need to shrink the ears, nose and mouth. For the smaller parts change the brush size to a smaller size. Shrink the ears, nose and mouth using the same technique as you used above. Now use the Clone Stamp Tool (S) to clone parts of the cheeks over the nose. You want it to look like the nose is erased. Use the Healing Stamp Tool to fix any not natural looking parts on the place where the nose first was.

Now you need to create alien eyes. Grab the Eraser Tool (E) and erase the eyes. Use the zoom tool to zoom in to get a better view of what you are working on. Go to Filter > Liquify and a new window will show. Use the Forward Warp Tool (W) in this window to stretch out the empty place where the eyes first were. I used a brush with a size of approximately 20 pixels. Stretch it until it has the shape of an alien eye. Press OK to accept the changes.

Double click the background layer to make it edible. Name it alien. Notice the lock that once appeared in the background layer no longer exist. Create a new layer by clicking the new layer icon at the bottom of the layer panel. Drag it under the Alien layer. Your layer palette should look like the one below:

Select the magic wand tool and click on the white out eyes to select them. Click the delete button to get rid of the white.

Save the alien eye shown below then open it in Photoshop.

Use the magic wand tool again and select the open eye in Photoshop. Copy the eye then click back on your work in progress document and click on the blank document below the picture and paste the eye there. Then paste another eye which will paste on a new layer. With the resize tool (Edit > Free Transform) resize both eyes to fit in the open eye space on the top layer.

Use the Burn Tool to make the areas around the eyes a little bit darker so it will look more realistic.

Now we need to add a green tint to complete the alien process. Click on Image-> Adjustments -> Hue/Saturation. Drag the Hue indicator to the right until you get a green hue you like. For an alien I think a slight yellowish-green will be great.

Drag the Hue indicator to the right until you get a green hue you like. The settings I used are shown below:

Here is my finished product.