Here is a technique I developed once. Take a long walk or drive and think of 5 main ideas about your subject matter. Yesterday I did this on a walk and came up with these 5. I recorded them using voice memo on my phone so I wouldn't forget what they were:
1) Lorca's theater is modernist. What does that mean? That he was searching for new forms of expression in the same way others were: Pirandello, Brecht... Both his experimental theater and his rural tragedies are modernist, though in different ways.
2) He is not expressing himself, his "self," but exploring multiple expressions of subjectivity, including ones that involve "blank" subjectivities.
3) His work can go in two directions: toward increased simplicity of motivation or toward ambiguity and complexity. For example, Yerma wants a baby but can't get one because she won't break social norms. E.g. having it with another man. It's not a complex psychology. Adela wants Pepe el Romano. At the other extreme are plays in which we don't know what the character wants.
4) Inseparability of his poetry and theater: see his theater as extension of his poetry in exploration of alternate subjectivities.
5) Lorca anticipates Beckett, but only if we liberate Beckett from the "absurdist" box and see his work in its complexity.
The second step is to find five ideas within each of these. You can take five more walks, or do it at your desk. Now you have 25. I recommend walking because you are exercising, avoiding sedentary scholar syndrome, and because the walking itself stimulates the brain. Don't worry if you only come up with 3 or 4 sub ideas; you will still have plenty of material.
Now what you want to do is to put the ideas in order, eliminating ones that aren't as pertinent. You might end up with 4 major ideas, with 4 subheading for each one. An idea that was once a major idea might be now a subheading of one of the others. You now have an outline of your argument. It is time to formulate a thesis.
Now find your textual examples; what plays you want to discuss, and in what order to support parts of your argument. You can bring in the secondary bibliography on the plays, etc...