So here's the thing.
I'm a junior-developer in a small company and have quite few experience working on big company projects. We have this old massive project which is not very well written. At all. A couple weeks ago I finished small cms project which lets you deploy sell sites. And now my manager assigned me to refactor this old project which is thousand times more complex then the one I developed to use the same concept as mine.
I have no experience managing other programmers, I don't know how are you supposed to separate tasks and how to plan all project till the end. I've never worked in a team where you have lead developer and who gives you technically explained tasks. Mostly it's just "place a button here to export this graph. And please be fast, it shouldn't take more then an hour." when in reality you only spend hour trying to figure out what tables to use and how this graph was created in the first place.

I'm overwhelmed and totally stuck.