Surface grinder with a magnetic base/table, very small cuts. Maybe run a green wheel (normally used for carbide).
Everything else is.....well.....ghetto....
Granted, other methods can work, you REALLY want a smooth flat finish. Otherwise, you can wipe out a cam lobe, then, you start all over.......for many valves......
I am not a machinist, I'm a decent "metal hacker". I have a few years working on my own and in shops.
I know when to "MacGuyver" and when to do it right.