I would always begin with a pencil and notepad to satisfy myself that the whole piece worked visually.This is the part I regard as "planning". Once the concept is there,you can move on to any software package that feels right for the way your mind works.I find it better to use a 3D model of the piece to make absolutely certain that it will all go together and that the joints all line up.You should then be able to export a .dxf of the parts to your CAM program-other file types might be possible but I have yet to have a failure with dxf.Some programs will allow you to do all this without the export and they eliminate the risk of losses in translation,which is a good thing because it isn't unknown for exported files to need several elements joined before creating toolpaths is possible.Rhino,mentioned above,is particularly good with it's associated CAM module but the cost is considerable.Have a look for a few demo programs and see which feels comfortable before committing.