I have found this step-by-step instruction on the web, which is quite simple to follow, and requires only 10min (I did it with my empty 128GB (SanDisk Ultra microSDXC).
All credit goes to the author of the above linked blog!
This works well and is close to what I have done. I have done additional steps to enable Metro apps to save to my MicroSD. Nook magazines and Kindle books take up a lot of space. If you don't use metro programs much, the above blog post whould be enough. If you want to use your Micro SD card for Metro apps, you would have to manually move the directory to the micro SD card, then recreate the directory as a Link using the mklink command. However, this has to be done in diagnostic mode (rebooting in repair -> Advanced Options -> Troubleshoot -> Advanced options -> Command Prompt ) so that Windows Metro itself never sees the directory as non-existent.
Caution: the above step is very dangerous if you click the wrong button. However, creating the wrong link only means it won't work, and the Metro app will recreate the directory on the main drive. You will have to redownload or transfer the contents back.
I normally only do this for the Metro data/document folders and not the app itself ( in the folder c:\Users\<username>\AppData\Local\Packages\<weird app directoryname>.
You can also change the default install directory in the registry, but that causes a lot of problems when you update the application.