Note the different types of clip on the key. The ones on the right in the photo should go into the trackpad side of the keyboard, where they sit in small holes. Put the bottom side of the key in, ensuring it sits in the holes.

You can see from the shape of the clips on the right why you have to take the key off from the top side. If you try from the bottom, you'll break the clip like I did on the bottom right in the photo.

Don't worry too much about broken clips. My space bar still works well despite 3/8 of the clips having been broken in the making of this guide. Hopefully you'll be more successful with the proper knowledge beforehand.