As movies have indicated, aliens have a huge brains so we 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 repeadetly 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 in step 3.

Now we need to create alien eyes. Pick the Bloat Tool again and make the eyes a little bigger.

To add a slight smile, click on the Warp Tool, select a quite small brush size and drag the mouth corners up to make him smile. Click OK to close the liquify dialog box.

Now we need to add a green tint to complete the alien process. Click on Enhance -> Adjust Color -> Adjust 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.