Use a format utility app to burn the Astroprint image onto a microSD card. There are a few options to consider, check out the guide on the RPi easy SD card setup.
With the SD card burnt with the Astroprint image, instert the card into the Pi. You need to connect a USB WiFi adapter to the Pi. You don't need to plug a keyboard+mouse and HDMI display to configure the Pi. The autoscript will create a wifi hotspot where you can connect to with your computer and then configure settings from a web browser.
1. Start the box and look for a wifi network called AstroBox-xxxx. (xxxx is 4 random numbers)
2. Remember this name if you don't plan to change it. This is how you access your box after setup http://AstroBox-xxxx.local
3. Get on that wifi.
4. Go to http://10.10.0.1
5. Perform setup and connect the box to your wifi.
Once the Astrobox is connected to your WiFi network, you may want to turn off the local hotspot - for security reasons!
Its under Settings > Internet > Connection > Turn OFF hotspot.
It's a public hotspot by default - Probably wouldn't want your neighbors controlling your 3D.
If your WiFi network doesn't apper in the list, you'll need to configure your setup so it broadcoasts public SSID - Hidden SSID isn't support (yet).
If your 3D printer isn't connecting to Astrobox, check your printers preferred buad rate. Use the "test connection" button and "re-scan" icon to update changes.
For any other questions, suggestions or feature requests. Check out the Astroprint forum.