It is not easy to answer theoretically without knowing the real situation. I would start to determine positive and negative issues from each system to frame what we want to keep, and what we want to avoid in the future integrated system.
Then, I would start integrating those clauses very similar in both standards like 9.3, 9.2, 7.5, 7.4, 7.3, 7.2, 5.2, …
In operations I would use the process approach from ISO 9001 and would try to include as much as possible the environmental operational procedures, instructions and records embedded in the daily life of the operation. Consider the central flow in this picture:
Organizations exist to serve clients. So, I recommend starting with modeling how the organizations serve clients based on the process approach and ISO 9001. Then, I consider other interested parties. Based on ISO 14001 and interested parties requirements I recommend organizations to determine environmental aspects and impacts, compliance obligations and risks.
From here, it is possible to determine what needs to be done to improve the interaction with the environment while serving clients. And what needs to be done can be translated to things like:
Add work instructions specifically about environmental practices
Update work instructions from quality with tips and requirements to be followed because of improvement needs relevant to the environment
Make changes in layouts and visual management in order to help people comply with environmental requirements while doing their work
Develop action plans to meet quality and environmental objectives