There are three phases of UbD: 1) Determine goals and objectives, 2) Establish what should be evidence of learning goals met (essentially the assessments, exams, and projects), 3) Lesson planning. The main difference with this design plan is that it ends with lesson planning when most teachers are likely to either begin with planning immediately, or shortly after determining standards and objectives. Most teachers would not think of planning exams, projects, quizzes, etc., before planning lessons.
The purpose of starting at the end is rather logical. Think about a trip you either have gone on, or would like to go to. Where do you start? Typically, you would start with your destination and then plan the trip. This is true with our teaching. If we determine from the beginning where our students are going, then we can better plan the journey to ensure they are prepared. Our initial planning of the goals, standards, and objectives allow us to determine what our students should be able to do. The evidence shows students have met the learning goals, so this needs to be well thought out (there is a book on UbD and they suggest ways to determine whether or not the evidence is strong enough).
Another important aspect of UbD is that units are based on big ideas. These are ideas that our students wouldn't automatically know. The big idea is not obvious and needs to be uncovered if the students will come to understand those big ideas.
UbD also emphasizes that we the teachers are the designers of our curriculum, not textbooks. In other words, textbooks should be used as a source, but they are simply a means to an end, not the end itself. Now, not every school is flexible, and some schools restrict teachers to specified curriculum. In that case, the teacher is very much limited.
UbD is easier when it is done as a team, preferably on a curriculum team. I, unfortunately, have been designing my curriculum alone. It began as a daunting task, but primarily because I was inexperienced in the beginning. After a year of using UbD in my own planning, I can't claim to be an expert, but I do claim to love it and agree with the ideas of needing to start from the end first. This ensures a journey that will lead your students to the desired destination.