This is a relatively simple task and the requirements are minimal.
The blank glass cut to size (2mm thick is ideal)
A sheet of Wet & Dry #200 to #400 grit
The grinding grit (one or two grades) #400 & #600 Silicon Carbide
A piece of glass as the grinding tool (blank) approx 2"x2" to 3"x3" (5x5 to 7.5x7.5 cm)
(I use 1/4" - 6mm thick glass as the grinding blank as it's easy to handle.)
The first task is to remove all sharp edges from the glass used as the grinding blank.
Place a few drops of water on the wet and dry and smooth off all edges & corners.
The same may be required with the blank screen, it's simple & quick.
Place the blank screen on some newspaper or similar on a flat surface, wet the back first - it stops the glass sliding.
Put a little #400 grit on the blank screen (about 20% of what's shown above) and add a few drops of water - this is a 5"x4" Crown graphic screen..
Now place the grinding blank on top and begin randomly grinding the screen.
After a while (2-3 minutes) add some fresh grit, a few drops more water, and start again.
Make sure that all parts of the screen are being ground and after the second grinding wash the screen well, dry and inspect.
Modern glass is very flat and not prone to hot-spots, but if you used an old lass plate to start with then you have to keep grinding until it's flat.
Now you have to decide on the final required fineness, for larger screens a final grind with #400 grit is probably all you require.
However for smaller screens a couple of further grinds with #600 grit gives a much smoother finer finish.
It's possible to grind a screen with just #600 grit or finer but the process is very much slower.
It's faster to grind with a coarser grit first then finely grind after.
Finally wash carefully to remove all traces of grinding past etc and dry, then inspect.
Re-grind if necessary.
The final screen should be as good as any commercially available glass screen, and much better than many older OEM screens.
Older screens can be improved easily by re-grinding them.
The total cost to make 10 screens, 1 10x8, 1 Whole Plate, 1 Half plate, 4 5x4's (Crown/Sped Graphic etc), 2 9x12's and a Quarter plate plus 3 grinding blanks, was £10 ($14) for the glass, cut to size and then £20 ($28) for the 2 grits.
As the there's more than enough grit left over for at least 20x more screens it's easy to see just how little each screen costs in real terms.
If I cut my own glass the total costs would drop even further.
Here's a 1898 take on the subject:
From The British Journal Photographic Almanac 1898
Ground Glass. --Rev. Arthur East says that there often appear in photo-graphic papers articles on substitutes for ground glass, it being little understood, probably, what an exceedingly easy process it is to make the very thing itself; and that starch, arrow-root, and suchlike things are almost, if not, quite as much trouble to utilise (and of not one-tenth the beauty and durability) as ground glass at home.
The following plan may, therefore, be acceptable :—
Take a clean negative glass of any size, and lay it on a flat, hard surface, such as a board or stone slab and sprinkle on this a pinch of emery powder, No. 1 (flour of emery will do, and do well, but the next quality, coarser, works more ‘ quickly). Lay on this a piece of broken glass about an inch square or there-abuts, and moisten the emery with a little water (do not use much so as to let it get 'sloppy,' and work all about).
Now work round and round all over the negative glass with the moderate pressure of two or three fingers until the gritty sound begins to go, which means that your emery is getting ground too fine (this will be in about a minute or two). Put on another pinch of emery, and work as before; in about ten minutes rinse your plate under the tap, and you will find probably with your finely ground surface a few ‘islands’ looking shiny, return the plate to the board, and work these patches out, and you will find a surface ground as finely as any you can buy, and there is no difficulty whatever in getting a perfectly even surface free from any scratches or defect whatever.
Any ironmonger will supply the emery, a pennyworth by post if you live ‘beyond the region of lamp-posts,’ and even the ordinary domestic knife powder will do, but it is too fine and works too slowly, and is inclined to be gritty and make deep scores in the surface of the glass.
If an extra finely ground surface is required as for a focussing glass, it only means rather longer grinding, and perhaps a little flour of emery to finish with; but the whole process can easily be done with one quality of emery, of which the best is probably the No. 1. If the surface of negative glasses were perfectly flat, it would be possible to grind two together, but the surface is never flat, and a small piece of glass is best to grind with. Some samples of glass are harder than others, an take rather longer to finish.
Substitute #400 grit Silicon Carbide for the coarse emery and #600 grit Silicon Carbide for the "Flour of Emery" finer grade, and everything else is the same :D