Elana Johnson, author of the soon-to-be released Control Issues, has a great breakdown on her blog of how to create a skeleton for your synopsis. The system is based on Dan Well's Seven Point System.
In a nutshell, write one sentence for each of these ideas:
Hook - how the story starts
Plot Turn 1 - introduces the main conflict - moves the story toward the midpoint
Pinch 1 - introduces the villain/pressure to force action
Midpoint - the point at which your story moves from beginning to ending
Pinch 2 - more pressure/problems, so many that the situation appears hopeless
Plot Turn 2 - the MC has what they need to overcome the main conflict - moves the story from midpoint to ending
