The Standing Band Ab Curl is a pretty cool exercise for your core that works a lot harder than you would think. Gravity really doesn’t give you a boost, since you’d be pulling against the resistance from above and/or to the rear. Typically you’d use a little over-the-shoulder yoke hooked to a cable and a pulley to a weight stack, like in a lat tower. I prefer to use bands crossed over my chest. Pretty cheap and simple, and you can adjust the resistance by stepping forward or back as needed.
Lean into the bands and use your core to curl forward into a “C” shape, pulling your chest toward your knees. This can be as hard or easy as you like, but be careful not to go too heavy with bands that are thicker than you can handle. If you can’t make the curl correctly, you won’t get the best benefit from it. I also arch backward a bit at the top, but let your own flexibility and condition of your lower back be your guide to your range of motion.
I have my bands girth hitched to my power rack, but you can hang them from your door frame, or stair railing or something else convenient. If you do things like this a lot, you might consider a pullup bar that fits over a door or in the frame for the best multi-tasking opportunities.
Go easy on your back, be safe and in control. Steady and slow is the right way to progress for these. I’d say anything between 20 and 50 reps would be good. If you can’t do 20, then step back or use longer or thinner bands. If you are comfortable with the power to do sets of only 10, then I’d say 3 sets with about a minute of rest between. FWIW I normally do 2 sets of 25.