what are the basic requirements for certification?
I don’t know if I’m understanding correctly your question. To be certified an organization has to comply with management system standards (both at the design of the system level and at the conformity of practices level) and has to comply with compliance obligations.
what are the best practices followed by organisations seeking for ISO 14k certification?
Setup a project sponsor, a project manager, and a project team. Determine the scope of the EMS. Ensure top management support, get training. Designing and implementing an environmental management system (EMS) implies being knowledgeable about ISO 14001:2015.
As a first step perform a Gap analysis, to determine the amount of work to be done - comparing what your organization already has in place versus ISO 14001:2015 requirements. From that GAP Analysis, you can develop your Project Plan, listing what needs to be done, by whom, until when.
From there it is implementation in order to close the gaps found. Then, perform an internal audit and the management review. There you can decide if your organization is ready for a certification audit.