The nut files should be gauged .002 larger than the string.
Therefore getting just the right gauge files for those particular strings can be challenging.
The general purpose files are really not suited for nuts.
Most people do it wrong anyway.
The slot should be
cut at the same angle as the head stock, not level with the finger board.
Cutting the slot level with the finger board is a really bad way to do it.
Even Stew Mac shows you the wrong way.
The nut slot should be cut like this: /
Not like this: ----
View attachment 47144 The high point of the slot is right where the nut meets the finger board.
Cutting the slot flat = sloppy intonation and poor tone of string.
But this is typical of cheap poorly made nuts.
(Yes, some people do it the right way)
Getting just the right tools is a slow process.
View attachment 47143
View attachment 47145
If the slot is cut wrong, the guitar will never play exactly in tune, but will always have problems.
This is why it takes skill and patience.
THIS:
If you don't have the right tools
If you don't have the patience and skill,
Then take it to a professional Luthier.
The nut slot if cut incorrectly, will totally screw up the tuning and intonation of the guitar, and it's not worth it.