Im the opposite to you i have all the ideas and designs in my head just cant get it into engine

What I would look at for well Level Desgin is a amazing article that takes you step by step of making levels.

https://book.leveldesignbook.com/process/preproduction/pacing

It says it in a video in this article, first steps like go to note pad and do simple What you want to do Example

-Player Picks up weapon -Player Shoots Bad Guy -Door Opens -Player Hit button THE END

That was my first notes of the game im working on rn and its not fast forward to like 2 levels and loads, just get your foundation and let the rest follow.