It's possible to grind nibs freehand, but it's very tricky to do. For many years, I've been using this simple device to sharpen and occasionally, "italicise" nibs.
I'd like to credit the author/inventor but I can't remember where I read about it. Like all clever ideas, it's very simple and very effective.
Here's how it's done :-
Cut a small block of wood to a size which fits comfortably in your hand. Cut a notch in one side. Fit a pen in the notch with the nib facing upside down. Hold the pen firmly in place with elastic bands. Allow it to protrude to taste - in other words, for a shallow shape, extend the pen out of the block - for a steep slope, pull it in a bit. Fix a sheet of fine glass paper, or similar to the surface. This is fixed to my drawing board, but a small sheet of glass is probably a better base. It is very important that the surface should be absolutely flat. - and that's about it.
Making sure that the wood is always in contact with the glass, move the block vertically away and towards you in a sawing motion on the glass paper just allowing the nib to touch. A few strokes back and forth will probably do the job. This little, ingenious device can either sharpen italic nibs or produce absolutely straight edges from round tipped nibs very quickly and with consistency.
When placed on the paper, the edge of the nib is parallel to the edge of the block of wood. I used only one elastic band in the picture, when I should have used one at the top of the block and one at the bottom. I wind them round the block and, as the pen is already set in the V slot there is absolutely no movement.
I occasionally use it with cheap fountain pens as experiments and would have no hesitation in using it on more expensive pens.
To recap - there are only two things of importance. Make sure that the pen is firmly anchored to the block and use a perfectly flat surface, preferably a piece of glass. You will find it very stable and almost impossible to rock to and fro.
I have been using this little device for a long time, and have lost very, very few nibs. In all these cases, the fault has been due to carelessness on my part.
In the attached photo, both nibs have been sharpened using the device. The tiny nib was produced from a much larger one. I ground down the sides freehand and then sharpened it using the device. As you can see, it's is capable of incredible accuracy, even at the tiniest nib size.
After grinding, I pour a small amount of liquid metal polish onto a piece of cardboard and describe small circles in it with the edged nib and my normal handhold. This results, very quickly, in a mirror-like surface. I have to remember to wash the nib thoroughly to remove the polish!