UI/UX design

Hello everyone!
I need your advice about UI/UX design.
When Should I start developing/maintaining UI/UX design working on an application?
Right now we are working on the main logic and there is rough UI/UX stuff already.
Only after the main logic is done we start working on UI/UX design?
I want to get when we have to start integrating nice and pretty buttons and stuff for UI/ux and think about it?
Thanks!