Getting Started on small circuit board design

[Elecmuso]

I have a simple circuit hat I would like to interface with an A20-OLinuXino-LIME2.

Warning, I'm a newby!

Is it common to base the design on an existing open source shield pcb design? The GPIO connectors are all there and the board is nicely laid out so I would just need to delete a handful of components and add a few (in my case mostly just ans ADC)?

If so how do I go about getting the files?

Do I get them from here?
https://github.com/OLIMEX/LIME2-SHIELD/tree/master/HARDWARE/Lime2-SHIELD-Rev.C

Have tried the schematic file but Eagle complains that the .sch file is not a Eagle file. How would I know what software was use to generate these files? Presumably they are not all compatible with each other.

Or should I start from scratch, or are there bare bones designs with connectors to the SBD already sorted for this sort of application?

[LubOlimex]

The shield is made with open source software - KiCAD. It is free to download and use. Make sure to use latest version.