The detail of a level depends on how much work and love you put into it. When making a room, you can always look up concept images, or just take a look around your house (if it's applicable), to get ideas. Look at the little stuff, look at the big stuff.
But first, I'd start off with just a basic map. Walls, ceilings, floors, doors, windows. You could draw on a piece of paper what you want the level to look like, and make sure the placement of rooms make sense. Then, it's just about sitting down to add all the details, and make it a realistic environment.
It's really just a matter of practice and patience. My first map is just a simple house with two floors, and five attached rooms. It took me almost 2 months to make!
Watch some custom stories on youtube, see how other people design their levels to get some ideas, and then just do it!! You'll never know what you can make until you really try!
A lot of people make their custom stories themselves. Doing the process alone can take months if you're story is decent and your maps are detailed. If you're still really unsure about making a map, the only thing I could suggest is seeing if someone would take an interest in your idea and possibly team up with you on it. :3