If the requirements are unclear then I can follow the below steps.

1. First of all,
I will prepare the review report with all clarifications/doubts about the unclear requirements.
2. Next, sent to the concerned persons like BA if available and get the required information.
3. If you (tester) have a direct contact with client then ask him directly via emails or phone, in my project we are using a common tool, we can post all our
doubts there and concerned persons will respond to them.