Making a Ground Glass Focus Screen
This is a relatively simple task and the requirements
The first task is to remove all sharp edges from the glass
used as the grinding blank.
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.
After a while (2-3 minutes) add some fresh grit, a few
drops more water, and start again.
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.
It's possible to grind a screen with just #600 grit or
finer but the process is very much slower.
Finally wash carefully to remove all traces of grinding
paste etc and dry, then inspect.
Older screens can be improved easily by re-grinding them.