There are different approaches, but here is what works for me:
Create a basic prototype of your idea that represents a “vertical slice” of the final game - meaning, create most of the functionality / gameplay features FIRST before you worry about the art and level design. Create a “training” level where all your game play features can be tested (climbing, swimming, AI test, etc). Do most of your work there and once you polish the gameplay then import your models, etc.
I also use this phase to create “tools” to help me design the levels easier. For example, if Im creating a platformer, then I create helper blueprints that can be used to create different types of platforms. I find this helps me make quick changes and when the time comes, I can create levels very quickly.
If you are interested in this process, check out my devlogs for my last game jam game - Verto Machina. If you watch the playlist you can see how I started with the basic idea and then flesh out gameplay on the same level, and eventually create the levels.
Hope this helps!