This will depend a lot on who the landowner is, what your plans are and what kind of relationship you have (if any).
It’s important to consider the perspective of the landowner and how your plans or ideas will impact them - positively and negatively. What are their wider objectives and how is what you plan to do going to help them? What are they likely to be concerned about and how can you allay their fears?
Also be really specific about your plans and realistic about what you might need to do on the site to achieve them. Will you need to provide water or electricity, or get planning permission for buildings? What kind of access and at what times?
If they are happy to work with you you will need some kind of legal agreement setting out each parties rights, expectations and responsibilities, and any fees to be paid. Exactly what kind of tenure you need will depend on what it is you are wanting to do, how much security you need, the timescales associated with your work and the level of investment of time and money you will be making. We have a guide to this here.
We have a guide specifically with securing access to land owned by local authorities here - but it’s probably worth a read no matter who the landowner is!