That's a very large question. Please read the book and it will give you an idea. As for hard, I would say it depends. I didn't have any trouble making the leap from OLTP designs to OLAP designs. But others, even good DBAs, have had troubles with the tables not being normalized in the traditional sense. And then others go to what I consider the other extreme, where they don't even use primary and foreign keys.

As for where you should begin, you should begin by working side by side with business users. Look at the reports that they are using and especially the spreadsheets that they are creating.