As someone who sews as a hobby, here's my suggestion. I don't know this Evangeline character, but I recognize the description of the boot detail you want. I'm actually planning to do a similar thing for my own boots when I cosplay Orphaner Dualscar from Homestuck (although his
boots will only go up to my knees, the principle for the fold-over is the same).
Cut a piece of fabric in the correct color and width (to get the width, measure around your leg where the fold-over will start, preferably while wearing your stocking boots so you can measure right around the top of them—measure both legs, as they could be slightly different in circumference). Add a little seam allowance to the width if your fold-over will be a solid piece or if you'll need to hem it, so you'll have room to do that bit of sewing.
Next, figure out how long your fold-over needs to be—then cut your fabric to the right size. It should be that measurement plus seam allowance (enough for one side if it's a material that doesn't need hemming, like felt, and enough for two if the material does need to be hemmed, like cotton weaves). If your fold-over needs hemming, you might want to do that first so it's easier to do. (A long stocking is hell to drag around a sewing machine.)
*If you need to hem your fold-overs' open sides, do it now!
Now, gently (without moving your stocking) push the fold-over material down under the stocking's upper edge by your seam allowance amount. Leave it sticking up for now! Carefully (so you don't stab yourself) safety pin it in place; about one every inch/2.5cm, because stockings like to de-stretch when you take them off. (I say use safety pins because normal pins are very easy to lose if you knock them out taking off the stockings.)
*If you need to sew your fold-over into a solid ring of fabric, now
is when you do that!
Once the fold-overs are pinned (and sewn/hemmed) and you've removed them, sew two lines of thread along the overlap of fold-over and stocking. One just
next to the very edge of the stocking top, and one just a teensy bit further away. The double lines of stitching will keep it more secure. If you have room and/or you're not sure if it'll be secure enough,
I might even run another line of thread along. More than three probably isn't necessary, though.
Now that the fold-overs are sewn in (but still sticking up) gently fold them down how they'll be for the cosplay. Tack them down with the teeniest stitches you can right below the top edge just to keep them folded down, in a thread color as close to the color of the fold-over material as you can get. (That's the thread color you would hopefully use for the whole process, but if nowhere else, use it here!) From there, you can add any necessary extra details (your X-thread, for example, or a lace trim if that were applicable... whatever it happens to be!).
And because you measured around your thigh, which is thicker than your calf (unless something is very strange), there should be no problem getting the fold-over that high up your leg.
I, personally, would want to use some sort of invisible/flesh-toned garters for something like this; you might want to keep that in mind, because if your fold-overs are heavy, they will
slid down your leg and you'll have to hike them up all the time. (A hook-and-eye set-up hidden underneath the fold-over, even attached to the seam allowance part of it, might be a good idea to attach them unobtrusively.)
I hope this helps! Let me know if you need any more explanation.