Gather information: decide a destination and start gathering relevant information about it through a good travel guide, websites and forums, where you can ask other travelers for advice on their experience in that destination. Then decide how long the trip will last and how much time you will spend in each place if you are going to visit several countries or a couple of cities or a place if it is a short trip.
Prepare a draft of the itinerary: prepare a draft with the time you will stay in each place, the things you can do or visit there and how you will move from one place to another. Always with flexibility in mind. Program only the days to travel and add extra days for contingencies, since there are many factors that could change your plans, such as a site you like a lot or nothing, someone to recommend something you did not know or the weather itself. And if your trip lasts more than a week, be sure to schedule a day off for each week of travel, as you may need it.