If you are asking how to cut roller blinds, then, I've cut them very carefully on a radial arm saw. I supose a power miter box would work too. You could take them to a hardware store that sells them and ask to have them cut down. If you cut them down yourself, remove the insert with the round pin in it first.
I usually just get them or take them to the hardware store. They have a machine and will even rewrap them for you for next to nothing. So long as hardware stores stay in existence, that is. Although, I think I heard the window treatment at HD can cut them too.