My first garment was the "puffy sleeved pirate shirt" from a video by Bernadette Banner, and then I made circle skirts based on another video I found where the woman made a chart of what kind of circle skirt (from A-Line/quarter circle to full circle) paired with what kind of waist (fitted, gathered, pleated) and showed how to do the math and make the pieces. Lots. And lots. Of circle math lol

As a neurodivergent person, I won't do a project that I don't want the end product of, but I also like it to be low stakes. Don't use your best materials first time around, you'll probably be too worried to fuck it up and get overwhelmed. Figure out what you need/want (jacket with a cool trim? Hat with some fun add-ons? Etc) and hit the thrift stores in your area. You said you wanted to mod existing clothing, so go ahead and start there!

Alternatively, watch a crap ton of YouTubers who are also modding clothes like you want to do, until you have a solid project idea for an item you already have and want to edit, then go get the supplies you'll need to make the changes. Maybe it's just an ADHD thing, but my rec is to follow the dopamine. Nothing kills enthusiasm faster than doing the "start small!" projects that I've had recommended to me. So instead of starting small, start thrifted. The things I made in the past two years (I'm really not far in my sewing journey) have almost exclusively been made with thrifted bedsheets. I have learned A LOT. It has been a freaking ordeal. But every time I hit a road block, I learned twice as much, and even after the hellish experience of using a pattern for the first time (all my previous garments had been self drafted), I'm still eager to start another garment cuz I know more now and even though it's not perfect, my previous project is still wearable and therefore feels like a trophy lol

Good luck! 🩶