Installing and using KiCAD for editing schematics, layouts, symbols, and footprints.
Download KiCad from here: http://kicad-pcb.org/download/
This also contains instructions for each system.
The KiCad tutorial is actually pretty good, so in general refer to it. The Avionics intro project (on Gitbooks) guide also walks through usage of KiCad.
To jump right in, go to Draw Electronic Schematics and then Layout Printed Circuit Boards.
Use the following link to learn how to make new symbols for components when you can't find an existing symbol for it in the KiCad libraries.
Often, it can be worth finding an existing symbol that is similar (for example, an older version of a sensor), copying it, and modifying it.
Try to make symbols following a functional pattern of placing pins. Symbols don't need to look like the footprint of an IC. Often, symbols of ICs will have all Vdds/Vccs/Vddios at the top, all Vss's at the bottom, and pins on the sides of the symbol.
STAR has has a repository hardware-sch-blocks which contains a library of symbols, star-common-lib. Create your symbols in this repository on a new branch, add them to the library, and when ready submit a pull-request. Make sure to update the datasheet link and description!
If you've made your own schematic symbol for a component, you will likely have to make a footprint for it as well. Footprints are described here. The following link will show you how to make new component footprints.
As with schematic symbols, try finding an existing footprint and then modifying it according to the actual component's datasheet. Datasheets will have drawings and dimensions of the footprint, often under a section such as 'Packaging'.
Many ICs come in standard packages (such as SOT-8). KiCad includes footprints for these standard packages, so often one can select one of these and then ensure with the datasheet that it matches--unfortunately, different manufacturers may use the same name but actually have slightly different footprints.
As with symbols, all STAR footprints go in star-common-lib in hardware-sch-blocks. Create your footprints on a new branch (makes sense to put them on the same branch as the new symbols), and submit a pull-request. Please let the current Avionics lead know when you submit a pull-request so it doesn't slip through their email.