Congratulations on your new van. You have a challenge with the bubble shape but with some cardboard templates you should be able to put the square pegs in the round holes. As Dieps said Youtube is your friend and Humbleroad is a great source for build inspiration. I would say figure out your wants and then figure out hot you can place them. Ventilation, insulation, wiring, plumbing are early on considerations. You can also do a partial build and use the van before finalizing the finished design. Camping in the empty van for a weekend or two. Weight is your enemy even if you have excess carrying capacity since weight will break things sooner, get you stuck and cost you in fuel economy. Bed frame rails from Ikea are light weight and strong and so is their spring slat bedframes at a cost that you cannot beat. Ikea cabinets are too heavy and I would recommend plywood or composite for cabinets over particle boards. Insulate your floor and work from the centre line down the floor.
Enjoy the journey.
2021 Ford Transit AWD ‘Iron Horse’
Formerly - 2007 Pleasure Way Plateau (06 Sprinter T1N), 1992 VW Eurovan Westfalia Hightop
Long ago - 1969 VW Westfalia & 1973 GMC Motorhome