The easiest way to install Android M would be of course to just flash an Android M rom. The roms are not quite here yet. You will need to flash. "D3VI0US" has put together an excellent tutorial which you will find below.
1-Download the preview factory image Downloads Android Developers
2-download this modified version of the stock M boot img https://goo.gl/SFWpMO
3-unpack the M preview .tgz file in your fastboot/adb folder and you will get the radio file and the bootloader file and the image-shamu-MPZ44Q.zip file
4-go to TWRP backup your ROM then wipe cache/dalvic/data/system ( watch out don't wipe the internal storage)
5-boot into bootloader and flash the bootloader : fastboot flash bootloader bootloader-shamu-moto-apq8084-71.11.img
6-fastboot reboot-bootloader
7-flash the radio file : fastboot flash radio radio-shamu-D4.01-9625-05.16+FSG-9625-02.94.img
8-fastboot reboot-bootloader
9-Unpack the image-shamu-MPZ44Q.zip file
10-you need just system.img from this file
11-flash the system : fastboot flash system system.img
12-flash the modified boot img that you downloaded in step 2 : fastboot flash boot boot-noencryption.img/B]
13-fastboot reboot
Via XDA