STEP 01: First thing I always do before starting my level is to make a simple 2D layout design on how the scene will be laid out. This is to help out a general idea what you aim for.
STEP 02: Next step is to make your layout into a 3D terrain form. Basic texturing for quick work flow. You want to get the layout done as quickly as possible, no side tracks of fancy effects.
STEP 03: Once you are satisfied with your layout, move onto next step, making roads, basic foliage, and texture verity.
BONUS STEP: Having a player scale reference always will help your scale of your level, think of them as your cyber best friend! Always implement them to any project.
STEP 04: As soon as you finish your basic setup. Be time for tweaking visual effects like sky, lighting, big noticeable static mesh details implemented.
STEP 05: Always add some sort of details where player could travel to, you want make the scene look beautiful when player explores, and moves to his objective. A dull scene may make the player feel bored.
STEP 06: Add in special effects, or changes into the scene, spice things up to make a story by the player just simply looking at it. A unique change can explain over a 1000 words of what events happen, and will keep the player engaged.
STEP 07: Implement final touches and little details with textures, meshes and etc. With level design a scene is never done, there is always little details you can implement no matter how much you put into it.
Final level design outcome. After level designer finishes the scene it'd be the environmental artists job to make additional 3D models, environment material textures and foliage. Additional enhancement for the scene.
A workflow of level design has taken place for a big battle, basic layout for the tutorial level to get used to the controls and layout.