To start off, you need to have an idea. If you want an facility, then you need some textures that may fit in the map and some colors such as : White (walls) Black (Floor/Outline) Any Light Color (Wall line) An example of facility is a map i made called “Facility Zone” Which is surpricingly good for a facility. Here is a picture
If you want to use vents like how i did, i recommend use them for gameplay or detail, detail should be out the way and collisions off. For gameplay use them like i did as in platforming! If you are doing facility and you want insane level, try do a bit of long, but not too hard jumps. Also try to make the parts longer and possibly faster lava to make the player want to be faster. You also might to add some jumps that have to make the player act quick or they die by lava. Also add metal ducts for detail in some places and you can even be creative and have your own ideas. This is the end of the tutorial, hope i help somebody.