Creating packages is usually your first step making a footprint, and it's also the most critical. Any mistakes here will mean your part won't fit, or your board may be unmanufacturable, so you if you're going to put some extra effort in somewhere, this is where you want it to be!
Before You Get Started
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.
1: Creating a New Package
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):
You can either select an existing package and edit it (just double-click an existing item in the list), or create a new package by entering a unique name in the 'New:' textbox towards the bottom:
Give it a sensible name that you'll remember if it's a one off part, or the standard name if it's a common package (see all the SOT varieties above).
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':