Start at the top. This way you don't need to support the portion you are not removing at the time. Cast iron pipe (the old stuff) is quite heavy. Doing it this way, you can cut the size pieces you want and can handle. There are 3 types of snappers. One has long handles and can be a pain in tight spaces. There are also ratchet handle cutters and hydraulic.
If you have never made a mistake, you haven't done much.