[Edit: I just realized this message was probably too late, but if not, here's some info anyway...let us know how the ride was!]

It's pretty easy to follow, since it parallels the north shore Metra line. I've only been on it once, probably when I was about 14 or so, and I had no trouble finding my way from Lake-Cook road south to Howard. Though some of that was on the Evanston lake front path, since as you say, the Green Bay Trail's southern end is in Winettka. I think there are (or at least were) on-street bike route signs linking the two trails.

There are some crushed limestone sections, and some breaks around Highland Park, but beyond that, you can basically ride the Green Bay Trail/McClory Trail all the way to Kenosha, WI if you so desired. There's also the North Branch Trail that goes through the Botanic Gardens which would make a nice alternative on the return if you wanted to make a loop.

Anyway, it might be time for you to bite the bullet and get a Chicagoland Bicycle Map (from the CBF), since it has excellent detail on all this stuff.