The code Mokha provided was in C, not ASM. If you’re on Windows you’ll need aMake.bat and a makefile, which will generate a lyn.event file (the compiled form of your c code). You can use Sacrifice.c as an example in the old skill system repo. However, in this case you don’t need to do that as Mokha has helpfully provided the compiled code already (the part with ALIGN, PUSH, POP etc). So you can just give that the name as your spellthief code, give it the extension .lyn.event and throw that in the same folder.
If you’re interested in learning more about C setup, Vesly has created a helpful guide here: C Setup for Dummies
But if you’re at that stage I would also recommend looking into the newer C Skill System if you want to make more skills.