This step should probably be called "Plan your time--but not too much." Ideally, your itinerary will give your trip structure, but not rigidity.
Sketch an itinerary. Try to make the concrete parts of your itinerary as realistic as possible--for instance, don't burden yourself with too many tight travel connections. The logistics of traveling take a lot of time and energy--often more than people plan on.
Aside from the nuts and bolts of non-changeable air or rail reservations, a non-touristy travel itinerary should be a flexible plan. Schedule your must-sees, hold on to your list of would-like-to-sees, and leave lots of time open for serendipity.
Plan to take it easy. If you are in any doubt about the amount of traveling or sight-seeing you'll be able do, plan on less. Any gaps will fill up naturally.