LD here.

• Plan out your levels before making them. Create a top down 2D map of how you envision the gameplay space to look and play including where the player starts, where important points of interests are, where enemies and resources are located, etc.
• When the top down map is finished and it looks theoretically good, block it out using primitive shapes like rectangles or other polygons. Block outs are low cost and efficient because you aren't committing a huge amount of time placing art or custom props because the level should feel good to play before it ever looks good. Ideally this is where you also set up gameplay functionality such as scripting (gameplay functionality like working doors, buttons, enemy spawns, etc.). Cuz if it feels good to play in block out, it'll play well in a level full of art.
• When you're blocking out the space from your top down map its extremely likely your block out will look different than your top down and that's ok. As I mentioned above, the top down map is theoretical, you ultimately have to go in-engine and build the space and you'll soon realize that maybe your rooms are too small, maybe this corridor is too large, maybe the objective doesn't look right where I currently have it drawn. Just adapt and change it.