The experimental approach should be outlined, not described in great detail. You will be expected to defend your approach during the examination. Every experiment, no matter how well conceived and performed, has its potential flaws and pitfalls. Your examining committee will be interested in determining if you understand the limitations and problems that you may encounter in your experiments. However, the final writing job is yours alone.
Good writing is difficult for everyone.
However, the ability to communicate scientific ideas, experiments, and results, both orally and in writing, is a skill that is essential for a scientist to have. It is a skill that comes only from practice and experience. There are books that provide some help. Also, your research advisor will be able to provide examples of well-written student proposals. You will learn a great deal from writing and defending this proposal.