We'll be creating a footprint for the GA1A1S202WP Light Sensor in this tutorial, since it's a nice easy part, and I just happen to have to make a breakout for this today. Make sure you have the datasheet opened so that you can follow along throughout this tutorial. The package details are found on page six of the PDF, marked as 4/15 in the footer.
Once you've opened up your library in the editor (just right-click on any library and select the 'Open....' option), you can create a new 'device', 'package' or 'symbol' via the three dedicated icons in the top left hand corner of your Eagle toolbar (shown in the same order below, and highlighted for convenience sake):
In this particular case, we"re going to call this footprint 'GA1A1S202WP' since it's a unique part, so we'll enter the part name in the 'New:' textbox, and then click OK.
After clicking OK you'll see a very sensible little dialogue box asking if you're sure you want to accept the consequences of such a monumental decision. Go ahead and click 'Yes':