It can be very difficult to figure out how to start a chapter (or any piece of writing). Instead of getting stuck on that, start by just brainstorming and jotting down some bullet points of the main subsections/topics/ideas for each chapter. In Word or any other typesetting software it is very easy to then arrange them in a logical order, start coming up with transitions from one idea to the next, and slowly build your writing around those ideas. The bullet points can be kept at the beginning of the chapter until you’ve ticked them all off. That way it might be easier to keep your focus on what you really need to say and cut out any unnecessary information that does not relate to the main ideas.
You can even start doing this while you’re doing your research. As you read up on the existing literature, jot down important ideas (and the publications they’ve come from) in your background/motivation chapter. When you’re developing your methods, make a note of every step (and changes as you modify your methodology along the way), including pointers to any relevant other work for that part of your methods. When you start to get results, maybe even copy and paste a graph or diagram of your findings into your thesis draft and follow by putting down a few bullets about what you think are the most important features in your data/results. By doing this you will save yourself valuable time towards the end, that otherwise you might have to spend trying to remember exactly whose paper it was that described the really important concept, or why exactly you set this parameter to the value you’ve used for your analysis, and not a different one.
Have a look at Study Hub's guidance about developing your points.