Making an effective audit

Answer
Effective means successful in producing a desired or intended result. What is the desired or intended result for your audit? What are the audit objectives? What have your client told you about the purpose of the audit? If the objective is mainly about conformity you should collect findings that will help you answer in your report if the company is conforming with performance targets and procedures or work instructions. I think you will need to check if:
inputs into production are identified, traceable and controlled;
people are competent;
production process is under control;
quality is under control;
production performance is under control;
non-conformities are identified and treated;
corrective actions are developed and effective;

And at the same time, you have to look into significant environmental aspects and impacts and check if they are under control, monitored and decisions made and implemented. Same for health and safety.

The following material will provide you more information about internal audits:
ISO 9001 – How to prepare for an internal audit - https://advisera.com/9001academy/blog/2017/09/26/iso-9001-how-to-prepare-for-an-internal-audit/
Five Main Steps in ISO 9001 Internal Audit - https://advisera.com/9001academy/knowledgebase/five-main-steps-in-iso-9001-internal-audit/
Free webinar on demand – How to perform an ISO 9001:2015 internal audit - https://advisera.com/9001academy/webinar/how-to-perform-an-iso-9001-2015-internal-audit-free-webinar-on-demand/
- Enroll for free course - ISO 9001:2015 Internal Auditor Course - https://advisera.com/training/iso-9001-internal-auditor-course/
- book – ISO Internal Audit: A Plain English Guide - https://advisera.com/books/iso-internal-audit-plain-english-guide/