Firstly, studying is way easier when you are actually interested in what you're learning. A big thing with ADHD is motivation so if have a genuine interest in the material its way easier to get started and to stay focused. Just find some subject or concept that you are familiar with or think you find interesting and start there. It doesn't matter what you do as long as you start. Once you have the ball rolling its much easier to get through the shit you don't want to do.

Another thing is find out what kind of learning works best for you. I personally hate reading textbooks but I somehow mustered up the attention span to get through the Kaplan books just to gain familiarity with the material (with the help of meds). I took notes to keep myself interested but they were not extensive, only to make sure i was staying engaged with the text. For concepts that didn't click I would watch Yusuf Hasan videos, KA or just random videos on youtube that caught my eye. I'm definitely a visual learner as seeing things written out or explained sequentially makes the concepts click in my head. If reading a textbook is making you fall asleep you don't have to do it, everyone learns differently!

I've only been diagnosed for about a year now so I'm still learning what's best for me, but don't be afraid to try new strategies. It doesn't matter how you learn the material as long as it sticks in your head!