Step 1 in any design is to really understand your use.– jracca1)
The “build” is the creation of your campervan from whatever condition it is currently in. Regardless of the eventual type of build, you may find benefit in using a standardized order of events.
Builds range from bare essentials to technical wonderlands to luxurious. There are no fixed lists of “Must Have's” or “Most Avoids”, because the whole point of building a van is that it has what you want. How fancy you get is up to you, from the most basic usage of a “metal tent” with nothing but a sleeping bag and a camping pad in the back, up to the $200k+ pre-made luxo-barges.
Here's some very rough estimations of build types, from the simple to the complex. Everyone can freely pick and choose the level of complexity of any sub-system that they want; there exist plain metal tents with multi-thousand dollar lithium battery setups, or finely crafted artisanal woodworking builds that have no more electricity than a few AA battery powered tap lights. It's completely up to you.
Note: These builds are targeted at people who are van-dwelling by choice. If your situation is different, please see Involuntary vandwelling.
Fast and simple, this is as minimal as it gets. You're in a metal box that keeps the elements off of you, and everything else is done in a similar way to backpacking. Also see frugal camper builds.
Good for casual overnights or weekends in campground near services, you're likely not making too many permanent modifications to the vehicle. You're still looking to be simple and efficient, but now you may take a few more of life's luxuries with you.
By now you're started to think you might like this whole van-life thing. You're going to spend multiple nights in it, maybe go a little farther and try your hand at the occasional boondock. You're going to start the build and have some tools, and abilities to make more permanent modifications. A little bit of interior build out can make things feel more put-together.
Starting to get committed now, and you're looking at spending a solid week+ at a time in the van. You may even be full-time, but keeping it to a budget. Modifications may be more serious/permanent, and you're doing actual construction. Maybe you're bringing your partner? The quality of life can go up a lot as you're capable of bringing more capabilities/comfort with you.
Some suggestions for longer month-ish trips focused on outdoor rec activities and looking good in staged photos
Suggestions for real full-time van living
More luxurious builds for the digital nomads tapping at a laptop
There's no “Right” way to plan. Various methods are used for planning: