Step 1: Define the germ

By this I mean, I write a one- or two-sentence "teaser" describing the book. Since I'm writing heroine-focused romantic suspense/action, I try to get into that teaser the following elements:

• the heroine's name and occupation (if occupation is important to the novel)
• her external quest
• her external roadblocks
• any time limit or special challenges

This is basically the back cover blurb if the book were to be published.

Here's an example from IN TOO DEEP:

Chris Hampton wants more than anything to gain full ownership of the charter yacht she shares with her drug lord ex-husband, and the latest cruise for one of her ex's handsome acquaintances looks like her financial salvation. What begins as a simple trip across the Gulf escalates into a cruise of terror as she and her mysterious passenger unravel layer upon layer of deception and intrigue, fighting their growing attraction while fending off storms, drug lords, and crooked DEA agents.

You can see that I've got only the barest bones to work with, but it's a premise and something to start with. Note that it has nothing resembling a plot point and only a smidgeon of a reference to Our Hero, with no fleshing out of him at all. We don't know at this point whether he's a good guy or a bad guy. We don't even know what the plot climax is going to look like.