What I did was buy some really simple black boots from payless. You might be able to still find some there (even try Ross, TJ Maxx, or marshels). I bought some black straps that were similar to the material the boots were made out of. And then, I hand sew them on. Also hand made the skulls with a pin in the back to pin it on to the boots.
Total costed less than $20
Atalanta (Fate/Apocrypha), Sailor Saturn (Crystal), Saber Alter & Dark Sakura (Fate/Heaven's Feel), Sangwoo & Yoonbum (Killing Stalking)