1. Get IPFire - I have downloaded it from http://downloads.ipfire.org/latest - I have selected the ALIX image http://downloads.ipfire.org/releases/ipfire-2.x/2.13-core75/ipfire-2.13.1gb-ext4-scon.i586-full-core75.img.gz which works fine on the APU1C
2. Install the image on your SD card / mSATA / USB / *you_name_it*
I have tried many media, especially various USB devices (3x sticks, 1x HD with Y cable, 2x powered HD and 1x powered HD over a powered USB hub). What I found out is, that using the original and the BIOS from April 1st, there is no reliable way of booting from USB. So I abandoned the idea and got myself a 16GB mSATA and a mSATA enclosure with USB connector. SD also worked, but I didn't like the idea as a firewall probably generates more logs than my RPis running from SD. Anyway, you need to get the image on your device. As I am using a Mac, here are the commands I am using. Please note that I am far from being an expert in these things, I just came to this procedure by googling.
Lists the devices currently mounted - be very careful identifying the right device - otherwise you can easily kill your OS by choosing the device you are running your OS from
diskutil unmountDisk /dev/your_device_here
Before writing the image to the device, you need to unmount it