There is no much big deal for importing here you go instructions:
1. It's recommended that you have latest WSB version possible (less mistakes to fix after importing the theme).
2. On your Win8/8.1 open your WSB, and click Open -> Import Windows 7 style,
3. After your theme is loaded, click save and save it to other location and close WSB,
4. Open again WSB with your new saved theme file and click test (sometimes it will crash on testing, after all WSB is still beta so it's normal behavior),
5. Now when your theme run on test mode, you will notice some or a lot of mistakes, now it's your job to fix those mistakes and after that your theme is ready to use on your win8/8.1 os (of course you need all default theme files to work with but i'm sure you know that), enjoy!