So you have a fantastic idea plot idea, and your muses are singing in your ears. So how do you actually get it down on paper?
Obviously, there are the given tropes- “Just write!” “Set aside time everyday!” But these don't work for everybody, and if you don't feel motivated at all, then doing those things becomes hard. It may be more helpful to try to figure out what kind of writer you are, and then understand what motivates you. From my observation, there are two major styles of writing.
First, there are the “Seat of the Pants” writers. I classify myself as one of these. These are people who do no revising as they write and don't plan out the course of the story when they begin much further than the setting, the characters, and the overall plot. Often, their story can get away from them, and they will add things as they get inspired. It is very difficult for them to write a synopsis without referring back to the original story because they don't remember exactly how they got where they did.
The drawbacks to this type of writing are many-you end up with a very unorganized mess of what may be loosely connected scenes that need to be divided into chapter chunks after the fact, or you may have scenes that just don't belong at all. Editing is a huge pain for this type of writer.
The plus side is that this type of writing is very easy to continue. Your momentum snowballs on itself. It doesn't feel like a chore, or an English writing assignment (now I must write the scene where Bob buys some staples). You're driving a very fast, very hard to control car, but it never really stops.
Of course, it's not all easy. The car can slow down. Seat of the Pants writers can write themselves into a corner and not realize it until they end up having to delete a good portion of their progress. When this happens, they feel the urge to give up. Also, if you end up writing too many scenes on a whim that don't fit the story, your desire to write can get choked.
If you're this type of writer, it's usually easy to start writing, but finishing can be hard. If you notice you have to struggle to keep writing, it may be because of the reasons above. Make sure you like where you story is going, and keep your characters the way you prefer them. A lack of enthusiasm may signify that you wrote something that doesn't really fit your story.
Also, Seat of the Pants writers often have several ideas that are in their head at once. As a result, they often don't finish anything. If this describes you, then really try to apply those basic tropes-write something every day. Have a schedule of a certain number of words per day, or at the very least a schedule of when you want to finish your novel. I solved this problem by specifying that I wanted a 75k novel done in a certain time. As a result I had to write 400 words a day. I let myself take breaks, however, and sometimes would write four times that amount in a day when the inspiration struck so I could take a break when I needed to. All the while I did battle other ideas that screamed to be worked on, but the urge wasn't as strong when I was focused so intently on the one story.
Another problem this type of writer has-often your idea stemmed from one really cool scene, or one really cool idea. If you really want to write that exciting scene in your head but you have to slog through a bunch of boring stuff first, don't just write the exciting scene and leave yourself with nothing but boring ones-lead up to it! Go in order. Don't let your excitement control you completely. Make that exciting scene something to strive toward, and reward yourself by writing it.
Now for the second type of writer-the organized writer. I am not one of these, so my advice to you guys may not be as helpful, but I'll try.
Organized writers tend to plan out their story in advance. They may write out a synopsis beforehand, or a chapter by chapter plan. Even if they don't, they know each twist and turn before they start. They often write out extensive character backgrounds so they know the reasoning behind every character choice. They think over every sentence. Their finished product typically requires less editing.
However, as a result of planning out everything, nothing new is left by the time they sit down to write. This kills motivation.
How to fix this? Some organized writers try to see the joy in the actual craft of writing. The fun is not in discovering the story, the fun is finding the best way to tell it. If you are having trouble staying motivated, try to think that way. Enjoy the craft, and keep the progress steady.
The set schedule for writing often works best for these guys too, and by that I mean sitting down at a specified time each day and writing for a certain duration. You already have your story planned, so there's no need to take advantage of bursts of inspiration.
Sometimes, though, those bursts of inspiration are needed, and it can be daunting to change a thoroughly planned idea. Look on those as a way to fit new ideas in a story, as a puzzle on how to make them fit. Control is good, but don't control the story so much that there is no wiggle room.
In sum, it is best to incorporate both elements of the two types of writing. Don't fly ahead with no plan at all and then give up when it's too hard, but on the flip side, don't over plan to the point that it's no longer fun anymore. Writing is a process, and it is by nature flexible. But it is also a structured art. Keeping that in mind will help your writing process, and knowing when to pull back and when to apply some needed control is important to maintaining that desire to write.