While I was working on my Valkyrie costume, one of the biggest challenges was her early ballet-slipper look, with the criss-cross straps that lace up her calves. While I did take a ballet class in college, we never got to the point of being fancy with shoes, so I had to do some research into how ballerinas keep those ribbons up while they're flexing their calves and leaping about. After I knew how it was done, I then had to make it work for me. All I sued were a pair of plain black flats, some elastic, and strips of black spandex (though if you have black elasic ribbon, you could use this as well)
Ihe first thing you need to know is that the straps are not attached to the shoes, as I found the movement of the shoe always put stress on the straps, so I said screw it. Instead, there's a loop of spandex under my foot. It still looks attached, but isn't! This way the straps could exist as a separate contraption from the shoes (which I like on their own and wear regularly anyway).
Then I made two bands with elastic, and covered them with black spandex. The one at my ankle actually has a snap on the back, but the one at the top of my calf is just a band that slides up, tight enough to stay put without cutting off the circulation
After that, I used two strips of spandex to criss-cross around my legs, and I connected them to the bands. Depending on how you criss-cross them, you may be able to let them stay as is, but for my thick calves (I'm a lot of girl to hold up!), I ended up attaching the bands together where they crossed on the side, and making thepieces along the back a flatter line (seen on my back leg in the picture).
Unless you have a cast of your leg that you can use, you'll have to do what I did and place the straps on while you're wearing them. Use pins to keep everything in place and then sew it down when you slide it off. Walk around a little in them in bare feet, up and down stairs, just to make sure there aren't any stress points you missed.
It's not the best explanation, but if you have questions, feel free to ask and I'll try to clarify it!