I personally would wait till they are at least 2 feet tall. I don't take mine till they are about 3 1/2 to 4 feet tall before the leaves start. All that you need is a flat shovel (not one with a curve) that is about 10 inches wide. Place it half way between the main plant and the pup. Push it straight down and you will feel where the corm connects to the pup. You want to keep pushing till it cuts all the way through. Then dig about a foot or so around the pup and when you are on your last spot gently lean the pup over. Be careful as you want as many roots as you can get. I usually plant it in a pot for a while so that it will be able to get a good root structure going. Good luck.

Hi, I always take most of the pups off when they are about a foot.
I always use a sharp knife, use your fingers to move a bit of the soil out of the way, then cut around making sure you get a bit of the corn and make sure you go deep enough around the pup to get some roots.
I also pot it up for about a month until it has plenty of roots.
Be careful of direct sunshine on the pup if its been a bit shaded as well.

Thank you both. Actually, having had another look at them they are about 2 foot tall already. I'll have a go at separating them (I think it's one of those things that is a bit daunting the first time you do it).