In message , Jon Pike
writes
"Dave S" wrote in
:

I know this sounds daft but the property of glass changes as it gets
older. The result of this is that it becomes more brittle and
therefore much more difficult to cut accurately. I support the advice
given by others, to ask a professional glazier to cut the glass for
you.

It's not old at all, it's brand new actually. I'm hoping I can avoid the
costs of paying someone else to do it.

Even if you do work out an effective technique, how much are you going
to waste in the learning curve? Offset the cost of a professional
against the glass wastage. You could always get an estimate and then
work out if its worth it having a go yourself.

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Paul reply-to is valid