" He and Ron headed downstairs from their dormitory together, - He and Ron walked down the stairs from their room discussing Angelina's idea that they were to work on a new move called the Sloth Grip Roll - they talked about Angelina’s idea to practice a new technique called the Sloth Grip Roll during that night's Quidditch practise, - at Quidditch practise and not until they were halfway across the sunlit common - when they were halfway across the sunlit room room did they notice the addition to the room - they finally noticed something new in the room that had already attracted the attention of a small group of people - a small group of people were already looking at the new thing in the room Hope this makes it understandable 🙂

First off, Harry Potter is written in a style that is not typical of American English, so it can be confusing even to native English speakers. So I applaud you for your efforts! You can actually break this into two sentences: " He and Ron headed downstairs from their dormitory together, discussing Angelina's idea that they were to work on a new move called the Sloth Grip Roll during that night's Quidditch practise. It was not until they were halfway across the sunlit common room in that they noticed the addition to the room had already attracted the attention of a small group of people. " Hope that helps!