There are two steps to turn your spare Raspberry Pi into an AstroBox.
1) Flash the AstroBox software image on an 8GB SD card.
Follow the instructions here to flash your SD card.
2) Setting up your Raspberry Pi AstroBox.
a) With the Raspberry Pi turned off, put in your AstroBox SD card and wifi dongle.
b) Follow the general AstroBox First Time Setup Instructions.