You will find some recommendations in these previous discussions:

https://www.researchgate.net/post/What_should_one_think_about_initially_before_enrolling_in_a_PhD_program/1

https://www.researchgate.net/post/How_to_write_a_brief_research_plan_for_a_masters_or_doctoral_thesis/1

https://www.researchgate.net/post/Best_ways_for_PhD_proposal_and_PhD_Thesis/1

https://www.researchgate.net/post/What-are-the-ingredients-of-a-good-thesis/1

https://www.researchgate.net/post/How_to_structure_and_write_a_Good_PhD_Thesis/1

https://www.researchgate.net/post/What_are_the_best_research_practices_during_Ph_D_life/1

https://www.researchgate.net/post/Is_publishing_during_your_PhD_worth_the_effort/1

https://www.researchgate.net/post/Should_I_necessarily_limit_the_Scope_of_research/1

https://www.researchgate.net/post/I_have_just_started_my_doctoral_What_are_the_some_good_things_that_i_can_focus_on_apart_from_publications/1

https://www.researchgate.net/post/According_to_your_experience_share_with_us_the_best_tools_for_writing_thesis_and_research_papers/1

https://www.researchgate.net/post/Hello-every-one-any-recommended-proofreading-service-website-for-thesis-or-paper-editing/1

https://www.researchgate.net/post/Role_of_research_supervisor_in_the_performance_of_a_PhD_student/1

https://www.researchgate.net/post/Do_you_think_that_PhD_supervisors_play_major_roles_in_polishing_the_research_skills_of_PhD_scholars/1

Make it as short, concise and elegant as possible :)

Some of the aspects to consider when writing thesis includes: research topic, problems, literature review, findings, materials and solutions.