The best way I have found is to remove the plant from the ground/pot wash the soil away so you can see where the pups attach and make your cut. I would also recommend that you take some of the mother corm with each pup you take. Trying to propagate musella lasiocarpa like regular musa will almost always result in cursing, frustration and failure.

The best way to have more success is to dig the whole thing up and separate them, like banana fun says. The other way is to wait until the pups are getting large, then separate them. Trying to separate small pups in-ground will almost almost always end up with no corm at the end of your cut-off pup. You really have to cut at an extreme angle, like probably around 45 degrees or less.