Also it's very important that the two images are on separate layers:
Now you need to make sure that the images line up, easy if you've used a tripod because the images should be exactly the same except for the exposure. After you've done that just erase the top layer (the foreground) to reveal the properly exposed sky:
Try to erase only the part you need, in this picture the sky, and after you've finished it you should have your properly exposed picture as shown in the first post
And there you have it you now have an evenly exposed picture that didn't require you to buy a filter. However it's not a perfect substitute for a grad-ND filter and definitely not as convient but it does the job for most scenes, and is great for people who can't afford or don't want to buy a filter.