This is a quick post to show you how to create custom library parts. The first thing to do is understand the Eagle library parts themselves. Below is a diagram that explains this pretty well.
There are three parts to an Eagle library part. The symbol is the schematic symbol that you would see in the schematic capture page of eagle, the *.sch file. The package is the device's footprint. This is shown in the board file during layout. When these two parts are created they are then combined to create a device which is the entity selected from the library when creating your schematic. Once the schematic is created and you change over to layout the footprints are automatically placed and connected. Now, lets create a library part.
First, create a library:
Once a library is created start creating your part. There is no order to creating the symbol or package, but both of these must be created before they are combined to make a device. So, for this example, lets create a schematic symbol. Open the library file you created and select the symbol button.
To place io pins use this button:
Use the "lines" button to draw the rest of the part. In this example, I just made two vertical lines representing a capacitor.
Last, lets add the text "value" and "name" to the schematic sheet. This will allow you to modify the name and value of the part in your schematic page. These are also added to the package or footprint file so these values show up in both schematic capture and layout. The attributes (text placed above) can be disabled in layout. The reason for this is because a lot of times in layout you won't want the details held in the value parameter on your silkscreen. To accomplish hiding this in layout, use the "smash" tool. This will allow you to select the text for both name and value and turn them off or "hide" them. Below is a screen shot showing the text button.
Next is the package or footprint:
To place a pad use the SMD pad tool:
To modify the pad size select the move tool, right click on the pad and there you can modify the properties of the pad.
As shown above, you can modify pad size and where the pad is placed. By modifying the x and y coordinates you can get very accurate placement of pads. Also, notice the Name and Value placed above and below the part which matches the text placed on the schematic symbol. The ">" sign denotes that it is not just text, but a parameter.
The last part is to combine the the symbol and the package to make the device. You can then choose the device from your library and place it in your schematic. This is pretty simple and doesn't need a screen shot. All you have to do is press the "device" button, enter a name and you will be taken to the device screen. In the device screen place the symbol by clicking the "add" button. Next, select the footprint by clicking the new button on the bottom right. Once this is done, press connect and connect your pins.
One thing to note is that the pin names all default to a numerical value, but you may want to right click on the pins and pads and edit the name so that it is a little more understandable. I know this was quick so let me know if you have any questions or would like to see a tutorial on something else.