DangerousPrototypes has written great tutorial how to make parts in Eagle Cadsoft software.
Eagle’s popularity is partly due to the sheer number of parts and footprint libraries available for it. Sometimes you can’t find the part you need in an existing library though, and it’s time to make your own. Making a new part in Cadsoft Eagle can be intimidating for new users, but the guide below shows how we make our own Eagle parts step by step. Alternately you can check it out on the Wiki.
A part is composed of 3 sections:
Symbol – the thing you connect to other parts on the schematic
Package – the footprint on the PCB that you attach the part to
Device – a symbol and a package together. Symbol connections are assigned to package pins