My trainer always makes me do a lap or two in two-point position in order to get my heels down. Once I sit, I am instructed to keep my leg and heels in the same exact place. Once you learn to stretch that muscle it’ll come easier! Throughout the ride my heels used to gradually come up, but every time that happened I corrected myself. Eventually your muscles become accustomed to it and your heels will be easier to keep down.
I've struggled with this in the past too. This might not be that helpful but I just kept thinking about my heels the whole ride. I also paid attention where my stirrups are. I kind of put them way infront of the stitching of my boots and this kind of forced me to keep my heels down.
Try excercises at home to stretch out your calves so it'll be easier for you to put them down. Something a trainer of mine did in the past when I was struggling was put rings on my feet (like the little toys rings for swimming) and the task was just keeping your heels down so the rings don't fall off. Might sound stupid but that was what finally drilled it into my head. 😂 Hope this kinda helped