I'm going to assume you've made your collections already...
Then, be at the bookshelf that's showing the book(s) you want to put into a collection. On the upper right side of the bookshelf, you should see an "Edit" button. Press that.
Next, tap any/all the books you want to move into a certain collection; a blue check mark should appear on each book. In the upper left of the book shelf should be a "Move" button. Press that and a drop-down should appear that lists your collection titles. Press the appropriate title where you want the books to go and they should move right in. You can repeat this however you wish, plus move books around various collections.
Hope this helps. Let us know how you get on.
Marilyn