As per my knowledge is concerned, following are required to register as a welfare society.
1) you need at least 25 members
2) land or money to donate to the trust.
3)An office with ownership documents or rental agreement
4)Objective of the trust
5) CNIC of the members
6) Affidavits from the members of the trust
You need to submit an application to the company registrar. Registrar is likely to visit your office. And also a copy of documents is required to be submitted to ISPR for further verifications.
after the completion of process, you will receive a Certificate of Trust and it will be enforceable under the Trusts Act 1882 or Registration Act 1860.
I recommend you to read the Act. It will help you further.
Rest ALLAH knows better.
regards,
zain Nadeem