Sit down with what you’ve written so far. Start at the beginning and read through. When you get to the second last page, open your notebook or word document. On the last page, get out your pen or put your hands on the keyboard. When you finish the last page, start writing.
The other thing I tell students is that you don’t have to write the thing consecutively. If you know that Martha and John are going to have a blowout over the dog, you can write that now, even if it won’t come for a few chapters. You may need to revise a bit more later, but sometimes writing out of order helps you focus on the big things and keeps the project moving.
A final tip - when you're writing something larger and know you're going to have stop, take five minutes and write a few lines about what you have to do next. You're in the moment of creation, and especially after a good session, you can see where you're going. But you don't always find your way back to that zone immediately, so giving the future you a little note about what the in-the-zone you saw coming can really help.