Short version: I usually bash my head on the keyboard for a while, then write a crappy synopsis. Then I try to rewrite it a couple of times. Then I try not to grimace as I send the damn thing out.
I don't know of a way to make it painless. Some writers chug the bourbon to be able to produce something. But then you get drunk, then you start writing a really bad synopsis, but then you end up writing about how nobody likes your writing anymore, and then you start crying, and then you fall asleep.
Not that this has happened to me.
But the head-banging on the keyboard might not be working so well. Regina Brooks, the literary agent, was to critique the first five pages of my Symphonians novel. But instead she said, "Girl, your synopsis is confusing as heck!" Bang bang and down went the novel in flames! Just because my synopsis is confusing as heck! Where's the bourbon!
So maybe that's why I'm so freaked out about sending her the book proposal. "Girl, your proposal is confusing as heck! Not to mention it's about dirt!" But I gotta remember that she did light up when I mentioned the gardening books. I've got to write the proposal up and stop freaking about it. Geez.
Maybe it would be a good idea if I went to Mo West and got my study room and just spent the day working on that book proposal. Then, in the query letter, I could also mention other gardening-book projects that are possibilities. Then we could take it from there. It beats sitting in the corner and whining.