Step 1 - be very very sure this is what you want to do. There is no turning back.
Step 2 - Cut or don't cut depending upon outcome of Step 1.
I would cut it below the green 'V' about where those two old branch/leaf scars are. My reasoning is that the cristata is rooted into the green Euphorbia and by cutting low, I am preserving some of those roots. That's just my theory, I don't really know.
Then let it dry and root like any other Euphorbia cutting.
