All the above is good info. You will get frustrated when tuning, but at some point the guitar decides to stay in tune, almost.
Another tip, use a piece of old string in each nut slot, as a "file". You could use the "next size up" string in each slot (if you have a 24 thou string, use a 26 - 28 thou, from a "heavier" set) , taking care to avoid deepening the slot, and avoid sloping the slot towards the fingerboard side - always a sharp edge at the fret side, slot sloping down towards head and tuners. Work it to-and-fro a while, you need to enlarge the slot just a bit. Good idea to always keep some old strings on hand.
Or, use lighter (thinner) strings, say a 9- 42 set if the guitar is stringed with 11-50's?
Or, start DIY with a home made bone nut, to your required specs. Looks good, sounds the best. Makes your guitar a "custom" build. Any old excuse to convince yourself to do it.