You might wanna enable root password, to make your life easier...instead of type sudo all the time, before every command.
Log in with osmc/osmc for user and pass, then type
sudo passwd root and type your new root passwd
Then log in as root with:
su root and your new password.
Then do all the installation stuff.
If i was you, i'd backup my pi SD card with this: https://sourceforge.net/projects/win32diskimager/files/Archive/
so if anything goes wrong, i can always restore back to this image.