Jay AnAm uploaded a video with 5 tips on designing levels in Unreal Engine. The thing is that you could actually use these tips to create levels in any engine.
What does he suggest? First of all, before opening up level designer like Unreal Engine you should define what you want to create. You can use paper and pencil or open up Photoshop and end up with the sketch. This way you will know how your scene will actually look like and it will be much easier to create your level.
Secondly, you should automate the actions that you execute all the time when creating your environments. For example, the shortcut keys. When you’re manipulating your objects, like moving them around, rotating them or scaling, you could use some buttons in toolbar, but it’s much more efficient to use the shortcut keys. You should focus on the things that are important like creating art, creating your scene and getting new ideas rather than searching for a particular tool’s location.
The next thing is to define whether you’re creating a game level or a scene for your portfolio. If you are creating a game level you have to keep in mind that every part of your landscape could be visited and explored by a character. And if you’re creating a scene from a certain angle, for a movie or something like that, you can forget about some parts of the design because they will never be seen.
The fourth tip is to go for realism. There should be a logic and some realistic purpose in your scene, so that it doesn’t look artificial. You should always focus on creating believable things.
Finally, don’t be afraid of trying new stuff. You can always model a new asset, but you can also use the things that you already have.