Can anyone share how to generate document ny flow in Omnistudio standard core. We are not using Omnistudio installed package.

I'm not quite sure what you're asking but if you're looking at doing document generation with omnistudio here's a fantastic playlist that takes you through it step by step. It has the older UI but it's easy enough to follow along and adjust for the new UI:  https://www.youtube.com/playlist?list=PLn7FvEG2ZtpGB5fpWUsxEFIQJjMKEdxEE