I can plan all sorts of specific aspects in a novel such as characters, locations, and the plot in general, but I struggle to actually write it the way it is in my head. How do I learn to write rather than just think about what I want to write?
It really comes down to two things, and nearly all professional writers recommend both of them: read a lot, and write even more. Just like anything else, you need to practice. Practice, practice, practice.
You need to develop your own unique voice, and the only way to do that is through trial and failure. See what works, see what doesn't, and if you persevere, you'll learn.
The relationship between planning and doing is a bit tricky, isn't it? I used to have a great deal of trouble with this, too. In my case, it was because I planned things that I didn't really know how to write. I would envision scenes in which someone dealt with a difficult, emotional situation that I have never experienced; scenes in which my characters would convey various informational tidbits to the readers (I wasn't quite sure how); or scenes that looked good in my outline but just wouldn't come together. For me, planning a novel was a little too much like making an overly-ambitious to-do-list (the kind that can never be completed because it isn't realistic!).
Of course, as has been noted in the other answers to your question, the solution was to practice. More experience writing was really a double solution: it helped me to execute more of my plans, and also to know how to make plans that are more realistically useful.
Several things helped me get the practice that I needed.
There are really two distinct challenges to writing:
If you are stuck emotionally, then you may feel as if everything you write is just a waste of time. If this is your challenge, then you simply must change the way you perceive the writing that you do.
Not Writing Is The Only Failure
Instead of considering it a failure to write poorly, begin to consider "not writing" your only failure.
The truth is that no matter how poorly you write, if you do in fact write, then you can begin to learn how to make things better. Of course, if you do not ever write, there will never be a way to evaluate whether or not you write well.
Many writers love language and have read great books and the emotional problem they have is that if they write and it turns out poorly then they have an idea in their mind that they will only have proven to themselves that they are not the actually great writers they dream of being. All day long they dream of sitting and writing but are crippled by this fear of failure. They start and stop continually without ever achieving anything significant in fear of proving to themselves that they are not great writers. This is too bad and is summarized beautifully in a great quote:
Much writing advice often concentrates on (and often becomes distracted by) emotional difficulties and unfortunately does not provide very much technical help.
A Great Thought Experiment
Imagine if a publisher contacted you and said,
I'm betting that you'd at least start in the 5th week and churn out 250 pages of something. Well, why do you need that motivation? Why not write 250 pages just to see how it feels? Why is it that the money would make you feel as if you weren't wasting your time?
For Help, Try This Book There are a number of books over the last 20 years that I've read that have actually helped me become a better writer. I may not be the greatest writer, but I am far better than I was 20 years ago, so it must be something. I must've learned something. One of those books is very old but has some great information that the author stumbled upon about how your brain works with writing. It's Becoming A Writer, by Dorothea Brande. Am I allowed to add links to books here? I hope so. Amazon link to Becoming A Writer, by Dorothea Brande
That book walks the reader through a lot of the emotional and has you set up Writing Appointments. You determine an exact time each day to write and then you watch yourself to see if you follow through. Basically, she says, if you never show up, your not a writer. Interesting and much more.
Technical Difficulties Now to the technical difficulties. If you are worried about your writing not being good and learning that you are not the Great Writer of Our Time, then you need to work on your technical ability so that you can be a better writer, so that people will actually enjoy your writing.
The first problem however, of getting technical help, is the problem of defining what great writing actually is.
I define it here as: Writing That Works (more in a moment on the inspiration of this phrase) But, what does that mean? That seems very loosely defined. That is why I further define Writing That Works as:
If you were to successfully achieve all of these with your writing, then it would be great writing -- and, of course, great reading.
Some quick notes on each of these.
Clarity If readers cannot tell what you're saying and are forced to re-read your sentences they will not suffer your writing long. Make it clear.
Appropriate If you are writing about the mating habits of the swallow tail butterfly in your novel about a bank robbery you are not writing a novel about a bank robbery. If you show a gun in the first scene, have it go off in the third scene - ala Chekhov's Gun. Show the reader, only what is appropriate for your story. Consider this at every moment as you edit your story and you'll get something done.
Stimulus / Response Writing Learned it from Jack Bickham's book, Scene & Structure (at Amazon). Once I learned this little gem I understood what to write and what not to write. Make things happen. When it happens follow the natural response. Continue on. A story will break out and your readers will be able to follow what is going on.
Smooth Transitions When you write the next thing that the reader is thinking because you've led the reader to think that, then you know you are writing great.
Music I hope you know that the sound that the words make in your readers' heads actually matter. That's why some people can write anything and you want to read it. It's about phrasing and sentence lengths and more. You can learn this stuff. Here's one of my recent short blog entries about this subject: The Song of Your Stories, The Music of Your Words
Style Many authors have such a strong voice you can tell it is them writing even without knowing who wrote a piece. Write like you talk. You can learn style by reading various authors and counting the words in their sentences, examining closely what types of words they use. And then finally learn like the artists of old, by copying what they've written. Try to get your sentences phrased so they sound like their sentences. It's a fun game and you'll learn a lot.
Desert Island Book
Finally, if I were marooned on an desert isle and could only have one writing book it would be the one that teaches all of this: Make Your Words Work, by Gary Provost - Amazon link.
Solve Technical, Resolve Emotional
Finally, as you concentrate on the technical you will find much of the emotional just slides away. Most likely that is because you are distracted by the work and learning but it is also because you'll become a better writer and you'll know it. As Norman Mailer famously said,
Once you trust yourself you'll probably find the block falls away.
I hope I've provided some solid ideas that'll help you move to the next level. I also blog about writing at: http://NewtonSaber.com/blog
Participating in National Novel Writing Month (November) helped me with this same problem. No excuses, no edits, no revisions, just write, for 30 days. I had a few friends who participated, and we supported each other in the Writers.SE chat room.
It helped me break out of the idea that I needed to compose in my head so that what I put on paper was the perfect expression of my thoughts. It helped me get over the notion that I always had to know the whole story before I could consider writing it. These were two major road blocks for me prior to two years ago.
I think you deserve some credit for being able plan and envision your story. When it comes to writing, you can be more of a Plotter, someone who outlines beforehand, or a Pantser, who is someone led by their gut feeling. A Google search yields some hefty results like this article that can give you the down low on how both approaches can help you get the best results.
That said, as far as translating the vision in your head onto paper, the journey of a writing a story begins with the first word. I recommend looking at Anne Lamott's "Bird By Bird: Some Instructions on Writing and Life" for some inspiration. Here are some great passages that I look to whenever I face a blank page and can't seem to start:
If perfectionism is your enemy, you can consider this passage:
Remember, it's okay not to get it right the first time:
In short, you just have to take it word by word. Without the words, there is no novel, regardless of how planned your story is.
I'd like to add to @Elliot's excellent answer by saying that it doesn't really matter what you write, so long as you just keep practicing. If you're reluctant to start writing your "dream project" because you want to do it justice, then work on a different project first. Try improvising a story, without planning anything about who the characters are or how it'll turn out--just start writing and see what happens. Do some other writing exercises or participate in workshops. Commit to NaNoWriMo. Or do what I did: keep a journal and write something in it, anything, every day.
Most importantly, don't get discouraged if you end up writing a lot of garbage--in fact, you should expect to. If you're challenging yourself enough to get better, most of your writing will be way worse than if you'd played it safe--but the stuff that's worth keeping will get much, much better.