Draw out a sketch of what you want your spats to look like. You may need more than one paper to do this.
This is what the basic design looks like when shown flat.
The most simple way to see if this design fits to your liking is to tape the ends together.
After this, you can flatten out your template and get to cutting it out of the fabric you want.
This is what it should look like. Although i would make the template have longer ends than the one i did here.
From here i made sure it looked okay before a sewed the bottom of it. I kept the back strings as is.
And your done! Another alternative to this fabric would be some more heavy materials such as vegan leather and such. In that case, i would recommend cutting the back and lacing up the back with some ribbon on string.