Lack of structure and the act of sitting down to study is extremely difficult for students with ADHD.
Be sure to have various places to study. One place might be a bedroom with a desk or a loft area with a desk, another could be the kitchen table, or the library. The key is to find a comfortable environment. Change up where you study.
Have a plan for which tasks need to be completed and when. Use a calendar to log your daily to-do list.
Make sure you have all the tools needed to study - paper, pen, pencil, notecards, highlighter.
Sit or stand to study - whatever works best for you.
Be in a quiet environment, use classical music as background noise, or use a noise cancellation devise.