There is a famous joke about Carnegie Hall:
A tourist, seeing a man carrying a violin case, asks "How do you get to Carnegie Hall?" The musician answers "Practice, practice."
In the case of Carnegie Hall, of course, there is a double meaning of "how do you get to…", because Carnegie Hall is both a literal location and a figurative goal. With a book, dissertation, or other writing project, there is no concrete location at which you need to arrive, so no one would ask "how do you get to your book?" But if the question is about the figurative goal--"how do you get your book to completion"--then the answer is the same: "Practice."
On on the Goodreads site, there are some standard questions that they ask of authors who are creating an author page; the questions are to help authors promote themselves. One of them is "How do you deal with writer's block?" Another is "What's your advice for aspiring writers?" My answer to both questions is: "Practice." And there's a third question--"How do you get inspired to write?"--for which a partial answer is practice.
Practice isn't an answer for everything, but it's an answer for a lot of things that ail the writer.
Practice--the right kind of practice--is my best answer for how to deal with writing block (though "writing block" is a terribly general description of a wide range of problems that all manifest with the same basic symptom--not writing, and not being able to write). The right kind of practices can help a writer become more comfortable in the writing process, which can alleviate many of the anxieties that can interfere with writing. And, as I suggested above, practice can help in finding inspiration, which is another cause of not writing/experiencing writing block.
Practice can help me find inspiration. Or at least it can help me manifest inspiration. For me, there is no lack of material to write about--I may lack confidence in what I can say about that material, but I have plenty to say. Many writers with whom I have worked have struggled with having too much to say: what they perceive as not having anything to write about often turns out to be related to having so many different things to say that they're blocked trying to choose among the many ideas. As I write this, I realize that I have a lot to say about what "inspiration" means--is it finding an idea? Or is it finding motivation? Or both? Or are both intertwined? One serendipitous outcome of my practice of writing this blog post is that I realize I have material and purpose--inspiration, if you will--to write a blog post about inspiration.
Given that I find practice useful both for finding inspiration and for dealing with writing block, it should hardly be surprising that "practice" would be the main recommendation that I would give aspiring writers. Which winds back to the musician and Carnegie Hall: the accomplished musician knows that the way to become an accomplished musician is to practice. And, to the extent that I am an accomplished writer, I know that the best way to become an accomplished writer is to practice.
NB: Some people have more talent than others, or at least, some people reach a given level of accomplishment more quickly and easily than others. And it can be really frustrating for a struggling writer to see someone else's success. But if you feel like you have a good reason for a writing project, you shouldn't give up on it because someone else learns writing more easily. The fact that you're struggling now, doesn't mean that you'll never improve (though writing is always challenging!). You get better with practice, and some ability as a writer may not manifest immediately. Anecdotal evidence of what I speak: my sister was, in school--through high school and likely college--much the better writer. She may still be a better writer in her own way, but I'm the one who has written and published a book, and co-written another published book. I may have needed more practice than she did, and she may well have had more talent than I, but that doesn't mean that my accomplishments can't stand on their own.