shape

Hello everyone. I have some mahaleb cherry and crabapple seedlings and want to give their trunks a dramatic shape while they are still flexible. What is the best season to do it: in spring or early fall? I live in a fairly cold climate, the temperature in greenhouse can drop to -5C. Heavy bends...

I'd like your thoughts and tips on how to improve the general shape of this young tree. What would you cut, leave grow, shorten, etc... Thank you! :-)