Dear Sumandar; you need to mention what kind of an. extension you want to plant, is it method of seeding, harvesting, marketing...? In that case you will get a good answer.
I think that one place to start, as well as to inform you as to what sorts of methods other researchers in the field are using, would be to consult current issues of some relevant scholarly journals, such as:
Selecting the appropriate agriculture extension method will provide you with an idea of whether to choose quantitative, qualitative, or mixed-method research. For instance, if it is a group approach, mixed-method would be appropriate.