Register and obtain your NTN on fbr.gov.pk
Get username and password. After that get login to iris.fbr.gov.pk and submit the registration form for tax filing year 2015-16. Once your registration is approved, you may submit the declaration form and your wealth assessment form for the year 2015-16.
But since you live outside and you don't pay taxes to Pakistan govt from your incomes, I am not sure if you need to file these tax returns or not. Because they are applicable on people having salaries of 400,000 PKR or more per annum. Get consultation from some tax consultant whether you need to file this or not. If I were in your place, I would just pay the additional 50k and wouldn't get into the hassle of filing tax as the process is quite hectic. (I am saying this just on the basis of the fact that you live overseas. Had you lived here in Pak, I would never have said this.)