Starting off you want to create a rough boxy shape and roughly outline it around your models leg.
This can be made with a box, going into edit mode, selecting 1 face and pressing E to extrude it. Keep doing this and position it around the leg until you have something similar to what I have on the left. You can stop half way up the leg if you do want them to cut off.
After you have created the rough shape around your leg, you will need to add a Subdivide Modifier to the box, set the Levels Viewport and the Render to 2.
Once you've added that, add a Shrinkwrap Modifier and set the target to the skin under the box. Make sure to set the offset to a reasonable amount off the leg, around 3 inch gap.
After you have set those, apply it by clicking the arrow pointing down and click apply.
Press Tab to go into Edit mode. Highlight all of the polys on the avatar by pressing A and at the top click on Edge > Un-Subdivide
In the bottom left you will have the Un-Subdivide settings come up, set the Iterations to 1
Go back into Object mode and re-open the modifier menu. Add another Subdivide Modifier and make sure the levels Viewport and Render is again 2.
Make another Skrinkwrap and set the target as the skin underneith again. This time do not add any offset to the Skrinkwrap.
Add another Modifier called Wireframe, set the Offset to full. Make sure the thickness is unticked. You can then change the Thickness to be however your preference is.
You might need to do a few tweeks to the mesh before actually applying these but this will give you a solid base to start off. Once done then you can apply all of them for the Fishnets to be complete. You can then weightpaint transfer from your model for the best results.
You should have some well made thigh highs then that can bend well with the skin. This method adds a lot more depth than just simply adding a texture to the legs but obviously is going to be more unoptimized than that method.
This is the same method that I used to make the fishnets on my upcoming model called NASA Baby.
I hope this has been helpful and let me know if you want any other tutorials
