Cutting and cleaning glass
The T-square is just that - a T-square for making sure that the glass is cut square, unless it has an integral glass cutter of course.

The tool they use is a glass cutter. It will have a diamond wheel or tip and scores the glass where it needs to be cut. Then a sharp tap will (hopefully) produce a clean break.

They should be easy to pick up at any hardware store. You might have more difficulty getting hold of a large enough T-square, but for a cold-frame you don't necessarily have to be 100% accurate.

Glass can be quite expensive. It would be worth getting a quote for buying the glass you would need and then compare to the cost of the tools you need to buy and your time.

However if (as chris points out in his comment) the glass is tempered then you won't be able to cut it. It will simply shatter if you try.