i assume you have installed line app on your pc running win8.
then you can login with your email and password on pc (the same login that you use on the mobile phone).
once you enter you email & password, there is 4 digit code come out. (valid for only 2,5 minute)
open your line application on your phone, go to menu MORE (...) \ SETTING \ ACCOUNTS \ EMAIL ACCOUNTS REGISTRATION.
you will be asked to type the 4 digit code.
finally, your application on pc will complete the installation you ready to use.