Once you have pulled everything out of the bus and made it water tight, you can start designing the type of layout you want as you will have more accurate measurement now. We used a free version of Google Sketch to make our plans. We found this programme easy to use with lots of tutorials online. Initially we took the measurements of the bus and drew out the empty shell. Save this as you may change our design and you need to come back to the shell. We ended up designing twice. The first was a very rough drawing of how we wanted to use the space. The second one was more of an evolving drawing that developed and changed as the build went along. We found it very useful to make separate components (these are like items instead of single lines) for the timber dimensions we had. Then you can cut and move and rotate these around your empty shell drawing, and effectively BUILD your internal framing in the drawing the same as you would in reality. This really helps by considering how all the various framing/wiring/plumbing will go together and how you can add various space saving elements into your design.
(TIP: work in mm in your drawings not in cm, as this will help increase precision)