Greetings & Welcome!
The easiest way is to simply move your new furniture in and secure it. Comfortable conversions can be extremely cheap and easy.
I have a regular household futon sofa/bed. Sofa by day, double bed by night.
I have a repurposed kitchen cabinet for my kitchen counter, but many people use those plastic drawers or dressers and just add a wood shelf on top for durability. Then I just use a camp stove, dishpans for sinks, and spray bottles for water, and ice chest for refrigeration, and your kitchen is complete, and you can also use everything outside if the weather is nice.
My bathroom is a handicap style, bedside bucket toilet. My shower is powered by a weed sprayer, with a kitchen sink sprayer nozzle replacing the original. A storage tote catches the shower water. My bathroom enclosure is a hula hoop for a shower curtain rod, and regular shower curtains. All of this can be usedd outside as well.
I bought my deep cycle house battery a junkyard for cheap, and it is charged while driving via an isolator. The battery has been placed in a marine style battery box, with multiple acc. outlets attached to the side of the box with double faced foam tape. I also have a cheapie inverter generator and household style car battery charger for the times that I’m not driving enough to keep my house battery charged.
Unless you’re planning on encountering extreme cold, your camp stove with a pot filled with either sand or dirt over one burner should keep you warm enough. In extreme cold, dedicated heaters are better. If using your stove, ventilation is required, and you should never run it while sleeping. A carbon monoxide detector is highly recommended.
A 12v fan blowing air out one window, will draw fresh in from all the other windows. In extreme heat, there are multiple ways to stay cool.
I think my entire conversion cost somewhere between $200 and $300 dollars, and that includes a dedicated heater and air conditioner. I can also give you directions for window screens or anything else you might need.
You didn’t mention what kind of van you have. If it’s a passenger van with lot of opening windows, you’re in luck, the interior is mostly done to start with hopefully. If it’s a cargo van, things just got very expensive and much more complicated. With a cargo van, you’re going to have to add insulation, paneling, roof vents, ceiling, flooring, and a whole lot more. Window vans are SO much better, and cheaper to convert.
I converted a cargo van once, and would never do it again. I’d trade it for a window van and save all that extra work and expense only to wind up with a very poor excuse for a camper van. Windows make life much more enjoyable.