How To purne Mothers Plants

Hello Everyone I don't know if this thread belong here but I'm growing them in flood and drain trays My question is I,m looking to get thick stems for me to clone how do i go about. What do i have to do to the mothers. I always get small stems I want thick thick steams thanks everyone again for your help and support

have a fan blowing on it so its all waving around like if it was outside in strong wind and that should make them a little thicker