Whether you are working from existing concept art or just designing the level from a blank slate, usually the first step is to ‘gray box’ your environment level, to start blocking out the overall shapes and composition of your level. Every artist is a little bit different, but I personally like to also start lighting work on the level while it’s in this graybox stage with simple assets, to start establishing the mood of the scene.
You can go a long way towards building the overall ambiance of an environment with your lighting setup, even if the level is made up of just simple cubes. After you are happy with the level design, layout, and initial lighting, I would look at your concept art or reference and try to determine which type of assets you will create that can be modular and re-usable in many areas. For example if you are working on a natural environment with a rocky cliff, you will find that a single cliff asset can be rotated around and re-used in multiple areas without the repetition being too noticeable. I would create these modular elements first, and start using them to replace the gray-box pieces with your new custom assets. You could opt to use assets from the Quixel megascans library to speed up the workflow for a natural environment, or choose to make everything yourself, the choice is yours.
After creating and placing all these modular pieces, I would then go in and hand-craft a few more bespoke custom elements to break up the repetition and modularity, and just continue refining the models, set dressing and lighting until the scene reaches the desired level of fidelity.