I've made a little guide on setting up a workspace to change the inside of the vessels and cockpits in KSP.
There are quite a few tutorials on this site but they all seemed either incomplete or too complex.
I just wanted to be able to open an IVA config file and move around some props, not make new ones !
There are a lot of superb IVA models around that could be made even better by adding RasterPropManager/MAS/ASET props to them.
So here is how I managed to set up an IVA workspace :