There is always an excuse to not write. Sitting down and allowing your imagination to work takes being still. And in this day and age, being still is not easy since we all want to accomplish so much. Writing a book takes commitment or you will never get to the end. If you don't get to the end, all you have is a dream. Making it happen takes drive and time in the chair.
Here are some ideas to make it happen for you:
1. First, decide if you're a plotter or a pantser. Do you want to plot out your novel? Or do your characters speak to you and tell you where to go? That's the first step.
If you're a plotter, you need to sit down and plot the entire book. Go ahead, you can fix it if it doesn't work, but this will tells you what your protagonist wants and what's he/she's up against.
If you're a pantser, you need to decide who your character is. What does she want more than anything in this world? What gets in her way? How will she overcome it?
If you're either, write a one-liner that tells you what your book is about. You can fix it later. You're not married to it.
2. Next: Set a word count. On a good day, how many words can you get to screen? Know your rate, and commit to an amount per week. If you can write 2,000 on a good day, commit to 10k for the week. That way, if you're short one day, you can write a little extra. The important thing is that you keep going no matter what. Life will always get in the way. Maybe you'll have to get up earlier. Maybe you'll have to take a pad and paper in the car while you're waiting at the dentist. Whatever you do, COMMIT to that word count.
3. Write that word count even if you don't have anything to say in that chapter. Because you know what? The best way to know what to write? Is to know well, that wasn't it. Trust me, fixing something and even ripping it out is better than having nothing to fix.
4. Get a writing buddy: Find a friend to challenge to say, 1,000 words in an hour. You'll be working at the same time, and the competition will ignite you.
5. Get a kitchen timer. Set it for fifteen to twenty minutes. Maybe thirty if you're feeling really creative. Do NOT do anything but writing until that timer goes off. No email. No phone calls. No laundry changeover. You are writing!
6. Guard your time. No writer can do it all. Say no when you need to. Take your dream seriously. Go on, get busy.