When I start working on a map, I first ask my player where they wanna go. This helps me flesh out future missions and story progression. In order to explore cool maps, I'm forced to ''railroad'' them a little bit, although, It is not a problem for my players. They can do anything within the boundaries of my maps. Besides, It's their story, not mine!

Find the Idea
First, I start with the motivation that leads my player in. This helps me write down the lore and side quest that might pick their curiosity. Then I find a particular mechanic that might add up to the dungeon. (This is not a requirement, but it adds a lot of uniqueness.)

Ex. .: In this case, player will be interacting with mine carts, to activate platforms or unlock doors along other items like explosive barrels. Warning, make sure that your fun gimmick is solution among other options when your player has to solve a puzzle. These gimmicks simply serve as fuel to their creativity. They should be used any way the player wants. 

Blocking
Once I have the Idea, I start sketching the map to help me flesh out the blocking. I try to highlight as soon as possible what are the most important locations. Once I'm satisfied, I create the polished version.

Render Pass
I use Dungeondraft to create my maps. I add details to my maps as I feel it need. After a while, I start taking notes over it to flesh out any zone that might be missing anything interesting elements. I try to find empty spots to add side quest information and journal log that might pick the interest of my players. It gives them a reason to explore. 

Side Content Breakdown
Once I'm satisfied with the map, I breakdown all the asset needed for the game, Journal, Traps, NPCs, Encounter Scenario, Reward, etc.