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The first step is to mock up the van's cargo area3) on the floor using painter's tape or other non-permanent method. The tape will prevent cheating and wishful thinking about what will actually fit in the space – the tape edge is the steel walls of your van.
Full size cargo vans usually have living spaces ~10ft for regular wheel base4) and ~12ft for long wheel base5). Extended body vans can be even longer. The cargo area is typically ~6ft wide. Mini-cargos have living areas around 6.5ft x 4.5ft. See actual interior dimensions for your intended platform.
If the “van” is taped off in a corner of a room you can also tape off the planned height of the cargo area by sticking tape on the wall[s].
Standing/sitting/lying in the tape “van” can help your mind start adjusting to the very real space restrictions you will face.
Once the van floorspace is established you can start moving real or mocked-up objects into it, and moving them around to see what works. It is easy and fast to learn what works at this stage; so work through any idea you might have.
Spend as much time as possible in the mocked-up van. Sleep in the bed. Set up your laptop and do your surfing/email/etc from there. It may seem silly but it will help your subconscious work on issues that remain, and bring good ideas to the surface.
If safety and ventilation allows, you may want to try cooking from the ingredients you have stored in your tape van's “pantry”. If the blinds are closed you may want to try out a sink- or wet-wipe bath.
You will become more and more aware of spatial relationships. Start carrying a tape measure so you can reality check any items you find at thrift stores, garage sales, or retail and are considering putting in the van.
Write down any particular dimensions you need to remember.