Glass trimming
Glass plants usually cut glass according to customer's order and this will be the best solution for you. However, if you decide to cut the glass yourself, you need to stock up on a special slice, soft substrate (table covered with a thick blanket) and a steel ruler.
If you want to trim the missing glass, measure the width and height of the window at several different points. The dimensions of the glass should be about 3 mm smaller than the smallest of the measurements. If the window is so crooked, that you have to cut out very irregular shapes, remove the entire window frame or draw a paper template and take it to the glass workshop.
Straight line cutting
Pull the slice with a movement towards you, pressing it with equal force. Break the glass along a wooden or steel ruler located under the scratch.