== To extract .xz file 
linux: tar xJf file.xz
mac: tar -xJf file.xz
windows: 7-zip

== To write image to sd card from linux
(umount before write)
umount /dev/sd[x]
dd if=image.img of=/dev/sd[x] bs=1M
sync

where x is the sd card device (find with fdisk -l)
use the whole device, not partition (correct: /dev/sdb, incorrect:/dev/sdb1)

== To write image to sd card from mac os x
(unmount before write and eject after write)
hdiutil unmount /dev/disk[x]s[y]
dd if=image.img of=/dev/rdisk[x] bs=1m
hdiutil eject /dev/disk[x]

where x is the sd card device (find with diskutil list)
use the whole device, not partition (correct: /dev/rdisk1, incorrect:/dev/rdisk1p1)

== To write image to sd card from windows
use Win32DiskImager
https://launchpad.net/win32-image-writer