Save yourself a headache and a ruined piece of glass.
Take it to a glass shop.
Probably the safest way and you'll make sure you get a proper job done.
How do they charge for their services? Thickness of glass or the amout of cutting?
They would charge on time taken, especially for just a cutting job when the glass is supplied already.
Be charming,and tell them what it is for and they may not charge anything.
Yep a small firm would likely be better. If you catch the chap with nothing much on at the time he may just do it on the spot and send you on your way.
If you do want to have a go yourself have your glass on a table or something solid rather than the carpet which gives a little. You can use a cloth sheet to avoid scratching the glass if you want. Mark it out beforehand to the correct size. Use a straightedge to make the cut neat. Apply firm pressure (but not excessive) to the cutting tool and, cutting away from you, make one solid cut from end to end of the glass and make sure you run off the end of the glass. You should hear it crackle nicely as you do it. DO NOT
go back over the cut again to make it deeper, you only need one cut, do it right first time. Now pick up the end of the glass holding both sides of the cut and tap the back/underneath of the cut from one end to the other with the end of a wooden hammer handle or similar, this will spread the cut stresses though the glass thickness. Tap gently over the whole length of your cut and if you get it right the pieces should just fall apart nicely.
Have a go on some spare scrap pieces first just to get the hang of it but it shouldn't be too hard.
If you want to clean up the sharp edges use a whetstone or carborundum sharpening block. You just need to knock the corners off the edge. Alternatively you can use some fine wet'n'dry paper on a block for flatting off car paint jobs.